diff --git a/src/archive/tar/tar_test.go b/src/archive/tar/tar_test.go
index cf8337c..1b211f8 100644
--- a/src/archive/tar/tar_test.go
+++ b/src/archive/tar/tar_test.go
@@ -94,13 +94,16 @@
 	var b bytes.Buffer
 	tw := NewWriter(&b)
 	hdr := &Header{
-		Name:    "file.txt",
-		Uid:     1 << 21, // too big for 8 octal digits
-		Size:    int64(len(data)),
-		ModTime: time.Now(),
+		Name: "file.txt",
+		Uid:  1 << 21, // too big for 8 octal digits
+		Size: int64(len(data)),
+		// AddDate to strip monotonic clock reading,
+		// and Round to discard sub-second precision,
+		// both of which are not included in the tar header
+		// and would otherwise break the round-trip check
+		// below.
+		ModTime: time.Now().AddDate(0, 0, 0).Round(1 * time.Second),
 	}
-	// tar only supports second precision.
-	hdr.ModTime = hdr.ModTime.Add(-time.Duration(hdr.ModTime.Nanosecond()) * time.Nanosecond)
 	if err := tw.WriteHeader(hdr); err != nil {
 		t.Fatalf("tw.WriteHeader: %v", err)
 	}
diff --git a/src/encoding/gob/gobencdec_test.go b/src/encoding/gob/gobencdec_test.go
index ecc91ee..41a06b2 100644
--- a/src/encoding/gob/gobencdec_test.go
+++ b/src/encoding/gob/gobencdec_test.go
@@ -746,7 +746,7 @@
 }
 
 func TestGobEncodeIsZero(t *testing.T) {
-	x := isZeroBug{time.Now(), "hello", -55, isZeroBugArray{1, 2}, isZeroBugInterface{}}
+	x := isZeroBug{time.Unix(1e9, 0), "hello", -55, isZeroBugArray{1, 2}, isZeroBugInterface{}}
 	b := new(bytes.Buffer)
 	enc := NewEncoder(b)
 	err := enc.Encode(x)
diff --git a/src/go/types/testdata/expr3.src b/src/go/types/testdata/expr3.src
index ab1a9f6..95d5c09 100644
--- a/src/go/types/testdata/expr3.src
+++ b/src/go/types/testdata/expr3.src
@@ -207,7 +207,7 @@
 	_ = time.Time{}
 	_ = time.Time{sec /* ERROR "unknown field" */ : 0}
 	_ = time.Time{
-		0 /* ERROR implicit assignment to unexported field sec in time.Time literal */,
+		0 /* ERROR implicit assignment to unexported field wall in time.Time literal */,
 		0 /* ERROR implicit assignment */ ,
 		nil /* ERROR implicit assignment */ ,
 	}
diff --git a/src/runtime/stubs.go b/src/runtime/stubs.go
index 107f260..616b716 100644
--- a/src/runtime/stubs.go
+++ b/src/runtime/stubs.go
@@ -241,8 +241,7 @@
 // in asm_*.s
 func return0()
 
-//go:linkname time_now time.now
-func time_now() (sec int64, nsec int32)
+func walltime() (sec int64, nsec int32)
 
 // in asm_*.s
 // not called directly; definitions here supply type information for traceback.
@@ -281,7 +280,7 @@
 func prefetchnta(addr uintptr)
 
 func unixnanotime() int64 {
-	sec, nsec := time_now()
+	sec, nsec := walltime()
 	return sec*1e9 + int64(nsec)
 }
 
diff --git a/src/runtime/sys_darwin_386.s b/src/runtime/sys_darwin_386.s
index 200961f..e911339 100644
--- a/src/runtime/sys_darwin_386.s
+++ b/src/runtime/sys_darwin_386.s
@@ -217,8 +217,8 @@
 	ADCL	$0, DX
 	RET
 
-// func now() (sec int64, nsec int32)
-TEXT time·now(SB),NOSPLIT,$0
+// func walltime() (sec int64, nsec int32)
+TEXT runtime·walltime(SB),NOSPLIT,$0
 	CALL	runtime·now(SB)
 	MOVL	$1000000000, CX
 	DIVL	CX
diff --git a/src/runtime/sys_darwin_amd64.s b/src/runtime/sys_darwin_amd64.s
index f944828..de8e9e3 100644
--- a/src/runtime/sys_darwin_amd64.s
+++ b/src/runtime/sys_darwin_amd64.s
@@ -198,8 +198,8 @@
 	MOVQ	AX, ret+0(FP)
 	RET
 
-// func now() (sec int64, nsec int32)
-TEXT time·now(SB),NOSPLIT,$0-12
+// func walltime() (sec int64, nsec int32)
+TEXT runtime·walltime(SB),NOSPLIT,$0-12
 	CALL	nanotime<>(SB)
 
 	// generated code for
diff --git a/src/runtime/sys_darwin_arm.s b/src/runtime/sys_darwin_arm.s
index 2c03c91..490a410 100644
--- a/src/runtime/sys_darwin_arm.s
+++ b/src/runtime/sys_darwin_arm.s
@@ -159,7 +159,7 @@
 	MOVW	R0, ret+12(FP)
 	RET
 
-TEXT time·now(SB), 7, $32
+TEXT runtime·walltime(SB), 7, $32
 	MOVW	$8(R13), R0  // timeval
 	MOVW	$0, R1  // zone
 	MOVW	$0, R2	// see issue 16570
diff --git a/src/runtime/sys_darwin_arm64.s b/src/runtime/sys_darwin_arm64.s
index c02d000..0e91d5b 100644
--- a/src/runtime/sys_darwin_arm64.s
+++ b/src/runtime/sys_darwin_arm64.s
@@ -151,7 +151,7 @@
 	SVC	$0x80
 	RET
 
-TEXT time·now(SB),NOSPLIT,$40-12
+TEXT runtime·walltime(SB),NOSPLIT,$40-12
 	MOVD	RSP, R0	// timeval
 	MOVD	R0, R9	// this is how dyld calls gettimeofday
 	MOVW	$0, R1	// zone
diff --git a/src/runtime/sys_dragonfly_amd64.s b/src/runtime/sys_dragonfly_amd64.s
index b950b69..f355268 100644
--- a/src/runtime/sys_dragonfly_amd64.s
+++ b/src/runtime/sys_dragonfly_amd64.s
@@ -148,8 +148,8 @@
 	SYSCALL
 	RET
 
-// func now() (sec int64, nsec int32)
-TEXT time·now(SB), NOSPLIT, $32
+// func walltime() (sec int64, nsec int32)
+TEXT runtime·walltime(SB), NOSPLIT, $32
 	MOVL	$232, AX // clock_gettime
 	MOVQ	$0, DI  	// CLOCK_REALTIME
 	LEAQ	8(SP), SI
diff --git a/src/runtime/sys_freebsd_386.s b/src/runtime/sys_freebsd_386.s
index 8b6ee1f..2c056fe 100644
--- a/src/runtime/sys_freebsd_386.s
+++ b/src/runtime/sys_freebsd_386.s
@@ -159,8 +159,8 @@
 	INT	$0x80
 	RET
 
-// func now() (sec int64, nsec int32)
-TEXT time·now(SB), NOSPLIT, $32
+// func walltime() (sec int64, nsec int32)
+TEXT runtime·walltime(SB), NOSPLIT, $32
 	MOVL	$232, AX // clock_gettime
 	LEAL	12(SP), BX
 	MOVL	$0, 4(SP)	// CLOCK_REALTIME
diff --git a/src/runtime/sys_freebsd_amd64.s b/src/runtime/sys_freebsd_amd64.s
index 158a60d..43aafe5 100644
--- a/src/runtime/sys_freebsd_amd64.s
+++ b/src/runtime/sys_freebsd_amd64.s
@@ -142,8 +142,8 @@
 	SYSCALL
 	RET
 
-// func now() (sec int64, nsec int32)
-TEXT time·now(SB), NOSPLIT, $32
+// func walltime() (sec int64, nsec int32)
+TEXT runtime·walltime(SB), NOSPLIT, $32
 	MOVL	$232, AX // clock_gettime
 	MOVQ	$0, DI		// CLOCK_REALTIME
 	LEAQ	8(SP), SI
diff --git a/src/runtime/sys_freebsd_arm.s b/src/runtime/sys_freebsd_arm.s
index 3c5a5cb..97aea65 100644
--- a/src/runtime/sys_freebsd_arm.s
+++ b/src/runtime/sys_freebsd_arm.s
@@ -166,8 +166,8 @@
 	SWI $0
 	RET
 
-// func now() (sec int64, nsec int32)
-TEXT time·now(SB), NOSPLIT, $32
+// func walltime() (sec int64, nsec int32)
+TEXT runtime·walltime(SB), NOSPLIT, $32
 	MOVW $0, R0 // CLOCK_REALTIME
 	MOVW $8(R13), R1
 	MOVW $SYS_clock_gettime, R7
diff --git a/src/runtime/sys_linux_386.s b/src/runtime/sys_linux_386.s
index 45320c0..5c5afac 100644
--- a/src/runtime/sys_linux_386.s
+++ b/src/runtime/sys_linux_386.s
@@ -151,8 +151,8 @@
 	MOVL	AX, ret+12(FP)
 	RET
 
-// func now() (sec int64, nsec int32)
-TEXT time·now(SB), NOSPLIT, $32
+// func walltime() (sec int64, nsec int32)
+TEXT runtime·walltime(SB), NOSPLIT, $32
 	MOVL	$265, AX			// syscall - clock_gettime
 	MOVL	$0, BX		// CLOCK_REALTIME
 	LEAL	8(SP), CX
diff --git a/src/runtime/sys_linux_amd64.s b/src/runtime/sys_linux_amd64.s
index 6ddcb30..be6f396 100644
--- a/src/runtime/sys_linux_amd64.s
+++ b/src/runtime/sys_linux_amd64.s
@@ -135,8 +135,8 @@
 	MOVL	AX, ret+24(FP)
 	RET
 
-// func now() (sec int64, nsec int32)
-TEXT time·now(SB),NOSPLIT,$16
+// func walltime() (sec int64, nsec int32)
+TEXT runtime·walltime(SB),NOSPLIT,$16
 	// Be careful. We're calling a function with gcc calling convention here.
 	// We're guaranteed 128 bytes on entry, and we've taken 16, and the
 	// call uses another 8.
diff --git a/src/runtime/sys_linux_arm.s b/src/runtime/sys_linux_arm.s
index 666b879..1712e9d 100644
--- a/src/runtime/sys_linux_arm.s
+++ b/src/runtime/sys_linux_arm.s
@@ -197,7 +197,7 @@
 	MOVW	R0, ret+12(FP)
 	RET
 
-TEXT time·now(SB), NOSPLIT, $32
+TEXT runtime·walltime(SB), NOSPLIT, $32
 	MOVW	$0, R0  // CLOCK_REALTIME
 	MOVW	$8(R13), R1  // timespec
 	MOVW	$SYS_clock_gettime, R7
diff --git a/src/runtime/sys_linux_arm64.s b/src/runtime/sys_linux_arm64.s
index 1b91b44..204aee7 100644
--- a/src/runtime/sys_linux_arm64.s
+++ b/src/runtime/sys_linux_arm64.s
@@ -182,8 +182,8 @@
 	MOVW	R0, ret+24(FP)
 	RET
 
-// func now() (sec int64, nsec int32)
-TEXT time·now(SB),NOSPLIT,$24-12
+// func walltime() (sec int64, nsec int32)
+TEXT runtime·walltime(SB),NOSPLIT,$24-12
 	MOVW	$0, R0 // CLOCK_REALTIME
 	MOVD	RSP, R1
 	MOVD	$SYS_clock_gettime, R8
diff --git a/src/runtime/sys_linux_mips64x.s b/src/runtime/sys_linux_mips64x.s
index 5a75bb8..a4bcc72 100644
--- a/src/runtime/sys_linux_mips64x.s
+++ b/src/runtime/sys_linux_mips64x.s
@@ -172,8 +172,8 @@
 	MOVW	R2, ret+24(FP)
 	RET
 
-// func now() (sec int64, nsec int32)
-TEXT time·now(SB),NOSPLIT,$16
+// func walltime() (sec int64, nsec int32)
+TEXT runtime·walltime(SB),NOSPLIT,$16
 	MOVW	$0, R4 // CLOCK_REALTIME
 	MOVV	$0(R29), R5
 	MOVV	$SYS_clock_gettime, R2
diff --git a/src/runtime/sys_linux_mipsx.s b/src/runtime/sys_linux_mipsx.s
index 73ce061..3b446b1 100644
--- a/src/runtime/sys_linux_mipsx.s
+++ b/src/runtime/sys_linux_mipsx.s
@@ -175,8 +175,8 @@
 	MOVW	R2, ret+12(FP)
 	RET
 
-// func now() (sec int64, nsec int32)
-TEXT time·now(SB),NOSPLIT,$8-12
+// func walltime() (sec int64, nsec int32)
+TEXT runtime·walltime(SB),NOSPLIT,$8-12
 	MOVW	$0, R4	// CLOCK_REALTIME
 	MOVW	$4(R29), R5
 	MOVW	$SYS_clock_gettime, R2
diff --git a/src/runtime/sys_linux_ppc64x.s b/src/runtime/sys_linux_ppc64x.s
index a40fe3b..b43bda1 100644
--- a/src/runtime/sys_linux_ppc64x.s
+++ b/src/runtime/sys_linux_ppc64x.s
@@ -157,8 +157,8 @@
 	MOVW	R3, ret+24(FP)
 	RET
 
-// func now() (sec int64, nsec int32)
-TEXT time·now(SB),NOSPLIT,$16
+// func walltime() (sec int64, nsec int32)
+TEXT runtime·walltime(SB),NOSPLIT,$16
 	MOVD	$0, R3 // CLOCK_REALTIME
 	MOVD	$0(R1), R4
 	SYSCALL	$SYS_clock_gettime
diff --git a/src/runtime/sys_linux_s390x.s b/src/runtime/sys_linux_s390x.s
index 47f34d9..2291718 100644
--- a/src/runtime/sys_linux_s390x.s
+++ b/src/runtime/sys_linux_s390x.s
@@ -169,8 +169,8 @@
 	MOVW	R2, ret+24(FP)
 	RET
 
-// func now() (sec int64, nsec int32)
-TEXT time·now(SB),NOSPLIT,$16
+// func walltime() (sec int64, nsec int32)
+TEXT runtime·walltime(SB),NOSPLIT,$16
 	MOVW	$0, R2 // CLOCK_REALTIME
 	MOVD	$tp-16(SP), R3
 	MOVW	$SYS_clock_gettime, R1
diff --git a/src/runtime/sys_nacl_386.s b/src/runtime/sys_nacl_386.s
index 05de20c..31e9b97 100644
--- a/src/runtime/sys_nacl_386.s
+++ b/src/runtime/sys_nacl_386.s
@@ -233,7 +233,7 @@
 	MOVL	AX, ret+24(FP)
 	RET
 
-TEXT time·now(SB),NOSPLIT,$20
+TEXT runtime·walltime(SB),NOSPLIT,$20
 	MOVL $0, 0(SP) // real time clock
 	LEAL 8(SP), AX
 	MOVL AX, 4(SP) // timespec
@@ -249,7 +249,7 @@
 	RET
 
 TEXT syscall·now(SB),NOSPLIT,$0
-	JMP time·now(SB)
+	JMP runtime·walltime(SB)
 
 TEXT runtime·nacl_clock_gettime(SB),NOSPLIT,$8
 	MOVL arg1+0(FP), AX
diff --git a/src/runtime/sys_nacl_amd64p32.s b/src/runtime/sys_nacl_amd64p32.s
index c2a24e8..db07ae5 100644
--- a/src/runtime/sys_nacl_amd64p32.s
+++ b/src/runtime/sys_nacl_amd64p32.s
@@ -242,7 +242,7 @@
 	MOVL	AX, ret+24(FP)
 	RET
 
-TEXT time·now(SB),NOSPLIT,$16
+TEXT runtime·walltime(SB),NOSPLIT,$16
 	MOVQ runtime·faketime(SB), AX
 	CMPQ AX, $0
 	JEQ realtime
@@ -268,7 +268,7 @@
 	RET
 
 TEXT syscall·now(SB),NOSPLIT,$0
-	JMP time·now(SB)
+	JMP runtime·walltime(SB)
 
 TEXT runtime·nacl_clock_gettime(SB),NOSPLIT,$0
 	MOVL arg1+0(FP), DI
diff --git a/src/runtime/sys_nacl_arm.s b/src/runtime/sys_nacl_arm.s
index 6cbc23f..aa3a098 100644
--- a/src/runtime/sys_nacl_arm.s
+++ b/src/runtime/sys_nacl_arm.s
@@ -196,7 +196,7 @@
 	MOVW	R0, ret+24(FP)
 	RET
 
-TEXT time·now(SB),NOSPLIT,$16
+TEXT runtime·walltime(SB),NOSPLIT,$16
 	MOVW	$0, R0 // real time clock
 	MOVW	$4(R13), R1
 	NACL_SYSCALL(SYS_clock_gettime)
@@ -209,7 +209,7 @@
 	RET
 
 TEXT syscall·now(SB),NOSPLIT,$0
-	B time·now(SB)
+	B runtime·walltime(SB)
 
 TEXT runtime·nacl_clock_gettime(SB),NOSPLIT,$0
 	MOVW	arg1+0(FP), R0
diff --git a/src/runtime/sys_netbsd_386.s b/src/runtime/sys_netbsd_386.s
index 8c4f004..5c15794 100644
--- a/src/runtime/sys_netbsd_386.s
+++ b/src/runtime/sys_netbsd_386.s
@@ -134,8 +134,8 @@
 	INT	$0x80
 	RET
 
-// func now() (sec int64, nsec int32)
-TEXT time·now(SB), NOSPLIT, $32
+// func walltime() (sec int64, nsec int32)
+TEXT runtime·walltime(SB), NOSPLIT, $32
 	LEAL	12(SP), BX
 	MOVL	$0, 4(SP)		// arg 1 - clock_id
 	MOVL	BX, 8(SP)		// arg 2 - tp
diff --git a/src/runtime/sys_netbsd_amd64.s b/src/runtime/sys_netbsd_amd64.s
index 7c7771b..c632a0b 100644
--- a/src/runtime/sys_netbsd_amd64.s
+++ b/src/runtime/sys_netbsd_amd64.s
@@ -169,8 +169,8 @@
 	SYSCALL
 	RET
 
-// func now() (sec int64, nsec int32)
-TEXT time·now(SB), NOSPLIT, $32
+// func walltime() (sec int64, nsec int32)
+TEXT runtime·walltime(SB), NOSPLIT, $32
 	MOVQ	$0, DI			// arg 1 - clock_id
 	LEAQ	8(SP), SI		// arg 2 - tp
 	MOVL	$427, AX		// sys_clock_gettime
diff --git a/src/runtime/sys_netbsd_arm.s b/src/runtime/sys_netbsd_arm.s
index a8914c1..789b12e 100644
--- a/src/runtime/sys_netbsd_arm.s
+++ b/src/runtime/sys_netbsd_arm.s
@@ -137,8 +137,8 @@
 	SWI $0xa001a9	// sys_setitimer
 	RET
 
-// func now() (sec int64, nsec int32)
-TEXT time·now(SB), NOSPLIT, $32
+// func walltime() (sec int64, nsec int32)
+TEXT runtime·walltime(SB), NOSPLIT, $32
 	MOVW $0, R0	// CLOCK_REALTIME
 	MOVW $8(R13), R1
 	SWI $0xa001ab	// clock_gettime
diff --git a/src/runtime/sys_openbsd_386.s b/src/runtime/sys_openbsd_386.s
index 76d22b0..b86875f 100644
--- a/src/runtime/sys_openbsd_386.s
+++ b/src/runtime/sys_openbsd_386.s
@@ -140,8 +140,8 @@
 	INT	$0x80
 	RET
 
-// func now() (sec int64, nsec int32)
-TEXT time·now(SB), NOSPLIT, $32
+// func walltime() (sec int64, nsec int32)
+TEXT runtime·walltime(SB), NOSPLIT, $32
 	LEAL	12(SP), BX
 	MOVL	$0, 4(SP)		// arg 1 - clock_id
 	MOVL	BX, 8(SP)		// arg 2 - tp
diff --git a/src/runtime/sys_openbsd_amd64.s b/src/runtime/sys_openbsd_amd64.s
index cf7a3fb..9a52e5d 100644
--- a/src/runtime/sys_openbsd_amd64.s
+++ b/src/runtime/sys_openbsd_amd64.s
@@ -180,8 +180,8 @@
 	SYSCALL
 	RET
 
-// func now() (sec int64, nsec int32)
-TEXT time·now(SB), NOSPLIT, $32
+// func walltime() (sec int64, nsec int32)
+TEXT runtime·walltime(SB), NOSPLIT, $32
 	MOVQ	$0, DI			// arg 1 - clock_id
 	LEAQ	8(SP), SI		// arg 2 - tp
 	MOVL	$87, AX			// sys_clock_gettime
diff --git a/src/runtime/sys_openbsd_arm.s b/src/runtime/sys_openbsd_arm.s
index f573a02..93a5d5b 100644
--- a/src/runtime/sys_openbsd_arm.s
+++ b/src/runtime/sys_openbsd_arm.s
@@ -150,8 +150,8 @@
 	SWI	$0
 	RET
 
-// func now() (sec int64, nsec int32)
-TEXT time·now(SB), NOSPLIT, $32
+// func walltime() (sec int64, nsec int32)
+TEXT runtime·walltime(SB), NOSPLIT, $32
 	MOVW	CLOCK_REALTIME, R0	// arg 1 - clock_id
 	MOVW	$8(R13), R1		// arg 2 - tp
 	MOVW	$87, R12		// sys_clock_gettime
diff --git a/src/runtime/sys_plan9_386.s b/src/runtime/sys_plan9_386.s
index 41aa2fd..6baa813 100644
--- a/src/runtime/sys_plan9_386.s
+++ b/src/runtime/sys_plan9_386.s
@@ -102,8 +102,8 @@
 	MOVL	$-1, ret_hi+8(FP)
 	RET
 
-// func now() (sec int64, nsec int32)
-TEXT time·now(SB),NOSPLIT,$8-12
+// func walltime() (sec int64, nsec int32)
+TEXT runtime·walltime(SB),NOSPLIT,$8-12
 	CALL	runtime·nanotime(SB)
 	MOVL	0(SP), AX
 	MOVL	4(SP), DX
diff --git a/src/runtime/sys_plan9_amd64.s b/src/runtime/sys_plan9_amd64.s
index 149505f..d7bd92c 100644
--- a/src/runtime/sys_plan9_amd64.s
+++ b/src/runtime/sys_plan9_amd64.s
@@ -92,8 +92,8 @@
 	MOVQ	AX, ret+8(FP)
 	RET
 
-// func now() (sec int64, nsec int32)
-TEXT time·now(SB),NOSPLIT,$8-12
+// func walltime() (sec int64, nsec int32)
+TEXT runtime·walltime(SB),NOSPLIT,$8-12
 	CALL	runtime·nanotime(SB)
 	MOVQ	0(SP), AX
 
diff --git a/src/runtime/sys_plan9_arm.s b/src/runtime/sys_plan9_arm.s
index d54f56f..f5c5e18 100644
--- a/src/runtime/sys_plan9_arm.s
+++ b/src/runtime/sys_plan9_arm.s
@@ -139,7 +139,7 @@
 	RET
 
 // time.now() (sec int64, nsec int32)
-TEXT time·now(SB),NOSPLIT,$12-12
+TEXT runtime·walltime(SB),NOSPLIT,$12-12
 	// use nsec system call to get current time in nanoseconds
 	MOVW	$sysnsec_lo-8(SP), R0	// destination addr
 	MOVW	R0,res-12(SP)
diff --git a/src/runtime/sys_solaris_amd64.s b/src/runtime/sys_solaris_amd64.s
index c542db3..aeb2e2c 100644
--- a/src/runtime/sys_solaris_amd64.s
+++ b/src/runtime/sys_solaris_amd64.s
@@ -354,8 +354,8 @@
 	CALL	AX
 	RET
 
-// func now() (sec int64, nsec int32)
-TEXT time·now(SB),NOSPLIT,$8-12
+// func walltime() (sec int64, nsec int32)
+TEXT runtime·walltime(SB),NOSPLIT,$8-12
 	CALL	runtime·nanotime(SB)
 	MOVQ	0(SP), AX
 
diff --git a/src/runtime/sys_windows_386.s b/src/runtime/sys_windows_386.s
index bd5de33..42583dd 100644
--- a/src/runtime/sys_windows_386.s
+++ b/src/runtime/sys_windows_386.s
@@ -432,8 +432,8 @@
 	MOVL	BP, SP
 	RET
 
-// func now() (sec int64, nsec int32)
-TEXT time·now(SB),NOSPLIT,$8-12
+// func walltime() (sec int64, nsec int32)
+TEXT runtime·walltime(SB),NOSPLIT,$8-12
 	CALL	runtime·unixnano(SB)
 	MOVL	0(SP), AX
 	MOVL	4(SP), DX
diff --git a/src/runtime/sys_windows_amd64.s b/src/runtime/sys_windows_amd64.s
index c61b79d..56079f6 100644
--- a/src/runtime/sys_windows_amd64.s
+++ b/src/runtime/sys_windows_amd64.s
@@ -465,8 +465,8 @@
 	MOVQ	32(SP), SP
 	RET
 
-// func now() (sec int64, nsec int32)
-TEXT time·now(SB),NOSPLIT,$8-12
+// func walltime() (sec int64, nsec int32)
+TEXT runtime·walltime(SB),NOSPLIT,$8-12
 	CALL	runtime·unixnano(SB)
 	MOVQ	0(SP), AX
 
diff --git a/src/runtime/time.go b/src/runtime/time.go
index 604ccde..9805753 100644
--- a/src/runtime/time.go
+++ b/src/runtime/time.go
@@ -301,3 +301,11 @@
 func time_runtimeNano() int64 {
 	return nanotime()
 }
+
+var startNano = nanotime()
+
+//go:linkname time_now time.now
+func time_now() (sec int64, nsec int32, mono uint64) {
+	sec, nsec = walltime()
+	return sec, nsec, uint64(nanotime() - startNano + 1)
+}
diff --git a/src/time/export_test.go b/src/time/export_test.go
index 38f2f45..26584b5 100644
--- a/src/time/export_test.go
+++ b/src/time/export_test.go
@@ -30,4 +30,6 @@
 var (
 	ForceZipFileForTesting = forceZipFileForTesting
 	ParseTimeZone          = parseTimeZone
+	SetMono                = (*Time).setMono
+	GetMono                = (*Time).mono
 )
diff --git a/src/time/format.go b/src/time/format.go
index b903e14..2da9a5e 100644
--- a/src/time/format.go
+++ b/src/time/format.go
@@ -424,8 +424,41 @@
 
 // String returns the time formatted using the format string
 //	"2006-01-02 15:04:05.999999999 -0700 MST"
+// 
+// If the time has a monotonic clock reading, the returned string
+// includes a final field "m±<value>", where value is the monotonic
+// clock reading formatted as a decimal number of seconds.
 func (t Time) String() string {
-	return t.Format("2006-01-02 15:04:05.999999999 -0700 MST")
+	s := t.Format("2006-01-02 15:04:05.999999999 -0700 MST")
+
+	// Format monotonic clock reading as m=±ddd.nnnnnnnnn.
+	if t.wall&hasMonotonic != 0 {
+		m2 := t.ext
+		m1, m2 := m2/1e9, m2%1e9
+		if m2 < 0 {
+			m2 += 1e9
+			m1--
+		}
+		sign := byte('+')
+		if m1 < 0 {
+			sign = '-'
+			m1 = -m1
+		}
+		m0, m1 := m1/1e9, m1%1e9
+		var buf []byte
+		buf = append(buf, " m="...)
+		buf = append(buf, sign)
+		wid := 0
+		if m0 != 0 {
+			buf = appendInt(buf, int(m0), 0)
+			wid = 9
+		}
+		buf = appendInt(buf, int(m1), wid)
+		buf = append(buf, '.')
+		buf = appendInt(buf, int(m2), 9)
+		s += string(buf)
+	}
+	return s
 }
 
 // Format returns a textual representation of the time value formatted
@@ -1022,11 +1055,11 @@
 
 	if zoneOffset != -1 {
 		t := Date(year, Month(month), day, hour, min, sec, nsec, UTC)
-		t.sec -= int64(zoneOffset)
+		t.addSec(-int64(zoneOffset))
 
 		// Look for local zone with the given offset.
 		// If that zone was in effect at the given time, use it.
-		name, offset, _, _, _ := local.lookup(t.sec + internalToUnix)
+		name, offset, _, _, _ := local.lookup(t.unixSec())
 		if offset == zoneOffset && (zoneName == "" || name == zoneName) {
 			t.setLoc(local)
 			return t, nil
@@ -1041,9 +1074,9 @@
 		t := Date(year, Month(month), day, hour, min, sec, nsec, UTC)
 		// Look for local zone with the given offset.
 		// If that zone was in effect at the given time, use it.
-		offset, _, ok := local.lookupName(zoneName, t.sec+internalToUnix)
+		offset, _, ok := local.lookupName(zoneName, t.unixSec())
 		if ok {
-			t.sec -= int64(offset)
+			t.addSec(-int64(offset))
 			t.setLoc(local)
 			return t, nil
 		}
diff --git a/src/time/mono_test.go b/src/time/mono_test.go
new file mode 100644
index 0000000..0bbb5c1
--- /dev/null
+++ b/src/time/mono_test.go
@@ -0,0 +1,252 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package time_test
+
+import (
+	"regexp"
+	"testing"
+	. "time"
+)
+
+func TestHasMonotonicClock(t *testing.T) {
+	yes := func(expr string, tt Time) {
+		if GetMono(&tt) == 0 {
+			t.Errorf("%s: missing monotonic clock reading", expr)
+		}
+	}
+	no := func(expr string, tt Time) {
+		if GetMono(&tt) != 0 {
+			t.Errorf("%s: unexpected monotonic clock reading", expr)
+		}
+	}
+
+	yes("<-After(1)", <-After(1))
+	ticker := NewTicker(1)
+	yes("<-Tick(1)", <-ticker.C)
+	ticker.Stop()
+	no("Date(2009, 11, 23, 0, 0, 0, 0, UTC)", Date(2009, 11, 23, 0, 0, 0, 0, UTC))
+	tp, _ := Parse(UnixDate, "Sat Mar  7 11:06:39 PST 2015")
+	no(`Parse(UnixDate, "Sat Mar  7 11:06:39 PST 2015")`, tp)
+	no("Unix(1486057371, 0)", Unix(1486057371, 0))
+
+	yes("Now()", Now())
+
+	tu := Unix(1486057371, 0)
+	tm := tu
+	SetMono(&tm, 123456)
+	no("tu", tu)
+	yes("tm", tm)
+
+	no("tu.Add(1)", tu.Add(1))
+	no("tu.In(UTC)", tu.In(UTC))
+	no("tu.AddDate(1, 1, 1)", tu.AddDate(1, 1, 1))
+	no("tu.AddDate(0, 0, 0)", tu.AddDate(0, 0, 0))
+	no("tu.Local()", tu.Local())
+	no("tu.UTC()", tu.UTC())
+	no("tu.Round(2)", tu.Round(2))
+	no("tu.Truncate(2)", tu.Truncate(2))
+
+	yes("tm.Add(1)", tm.Add(1))
+	no("tm.AddDate(1, 1, 1)", tm.AddDate(1, 1, 1))
+	no("tm.AddDate(0, 0, 0)", tm.AddDate(0, 0, 0))
+	yes("tm.In(UTC)", tm.In(UTC))
+	yes("tm.Local()", tm.Local())
+	yes("tm.UTC()", tm.UTC())
+	yes("tm.Round(2)", tm.Round(2))
+	yes("tm.Truncate(2)", tm.Truncate(2))
+}
+
+func TestMonotonicAdd(t *testing.T) {
+	tm := Unix(1486057371, 123456)
+	SetMono(&tm, 123456789012345)
+
+	t2 := tm.Add(1e8)
+	if t2.Nanosecond() != 100123456 {
+		t.Errorf("t2.Nanosecond() = %d, want 100123456", t2.Nanosecond())
+	}
+	if GetMono(&t2) != 123456889012345 {
+		t.Errorf("t2.mono = %d, want 123456889012345", GetMono(&t2))
+	}
+
+	t3 := tm.Add(-9e18) // wall now out of range
+	if t3.Nanosecond() != 123456 {
+		t.Errorf("t3.Nanosecond() = %d, want 123456", t3.Nanosecond())
+	}
+	if GetMono(&t3) != 0 {
+		t.Errorf("t3.mono = %d, want 0 (wall time out of range for monotonic reading)", GetMono(&t3))
+	}
+
+	t4 := tm.Add(+9e18) // wall now out of range
+	if t4.Nanosecond() != 123456 {
+		t.Errorf("t4.Nanosecond() = %d, want 123456", t4.Nanosecond())
+	}
+	if GetMono(&t4) != 0 {
+		t.Errorf("t4.mono = %d, want 0 (wall time out of range for monotonic reading)", GetMono(&t4))
+	}
+
+	tn := Now()
+	tn1 := tn.Add(1 * Hour)
+	Sleep(100 * Millisecond)
+	d := Until(tn1)
+	if d < 59*Minute {
+		t.Errorf("Until(Now().Add(1*Hour)) = %v, wanted at least 59m", d)
+	}
+	now := Now()
+	if now.After(tn1) {
+		t.Errorf("Now().After(Now().Add(1*Hour)) = true, want false")
+	}
+	if !tn1.After(now) {
+		t.Errorf("Now().Add(1*Hour).After(now) = false, want true")
+	}
+	if tn1.Before(now) {
+		t.Errorf("Now().Add(1*Hour).Before(Now()) = true, want false")
+	}
+	if !now.Before(tn1) {
+		t.Errorf("Now().Before(Now().Add(1*Hour)) = false, want true")
+	}
+}
+
+func TestMonotonicSub(t *testing.T) {
+	t1 := Unix(1483228799, 995e6)
+	SetMono(&t1, 123456789012345)
+
+	t2 := Unix(1483228799, 5e6)
+	SetMono(&t2, 123456789012345+10e6)
+
+	t3 := Unix(1483228799, 995e6)
+	SetMono(&t3, 123456789012345+1e9)
+
+	t1w := t1.AddDate(0, 0, 0)
+	if GetMono(&t1w) != 0 {
+		t.Fatalf("AddDate didn't strip monotonic clock reading")
+	}
+	t2w := t2.AddDate(0, 0, 0)
+	if GetMono(&t2w) != 0 {
+		t.Fatalf("AddDate didn't strip monotonic clock reading")
+	}
+	t3w := t3.AddDate(0, 0, 0)
+	if GetMono(&t3w) != 0 {
+		t.Fatalf("AddDate didn't strip monotonic clock reading")
+	}
+
+	sub := func(txs, tys string, tx, txw, ty, tyw Time, d, dw Duration) {
+		check := func(expr string, d, want Duration) {
+			if d != want {
+				t.Errorf("%s = %v, want %v", expr, d, want)
+			}
+		}
+		check(txs+".Sub("+tys+")", tx.Sub(ty), d)
+		check(txs+"w.Sub("+tys+")", txw.Sub(ty), dw)
+		check(txs+".Sub("+tys+"w)", tx.Sub(tyw), dw)
+		check(txs+"w.Sub("+tys+"w)", txw.Sub(tyw), dw)
+	}
+	sub("t1", "t1", t1, t1w, t1, t1w, 0, 0)
+	sub("t1", "t2", t1, t1w, t2, t2w, -10*Millisecond, 990*Millisecond)
+	sub("t1", "t3", t1, t1w, t3, t3w, -1000*Millisecond, 0)
+
+	sub("t2", "t1", t2, t2w, t1, t1w, 10*Millisecond, -990*Millisecond)
+	sub("t2", "t2", t2, t2w, t2, t2w, 0, 0)
+	sub("t2", "t3", t2, t2w, t3, t3w, -990*Millisecond, -990*Millisecond)
+
+	sub("t3", "t1", t3, t3w, t1, t1w, 1000*Millisecond, 0)
+	sub("t3", "t2", t3, t3w, t2, t2w, 990*Millisecond, 990*Millisecond)
+	sub("t3", "t3", t3, t3w, t3, t3w, 0, 0)
+
+	cmp := func(txs, tys string, tx, txw, ty, tyw Time, c, cw int) {
+		check := func(expr string, b, want bool) {
+			if b != want {
+				t.Errorf("%s = %v, want %v", expr, b, want)
+			}
+		}
+		check(txs+".After("+tys+")", tx.After(ty), c > 0)
+		check(txs+"w.After("+tys+")", txw.After(ty), cw > 0)
+		check(txs+".After("+tys+"w)", tx.After(tyw), cw > 0)
+		check(txs+"w.After("+tys+"w)", txw.After(tyw), cw > 0)
+
+		check(txs+".Before("+tys+")", tx.Before(ty), c < 0)
+		check(txs+"w.Before("+tys+")", txw.Before(ty), cw < 0)
+		check(txs+".Before("+tys+"w)", tx.Before(tyw), cw < 0)
+		check(txs+"w.Before("+tys+"w)", txw.Before(tyw), cw < 0)
+
+		check(txs+".Equal("+tys+")", tx.Equal(ty), c == 0)
+		check(txs+"w.Equal("+tys+")", txw.Equal(ty), cw == 0)
+		check(txs+".Equal("+tys+"w)", tx.Equal(tyw), cw == 0)
+		check(txs+"w.Equal("+tys+"w)", txw.Equal(tyw), cw == 0)
+	}
+
+	cmp("t1", "t1", t1, t1w, t1, t1w, 0, 0)
+	cmp("t1", "t2", t1, t1w, t2, t2w, -1, +1)
+	cmp("t1", "t3", t1, t1w, t3, t3w, -1, 0)
+
+	cmp("t2", "t1", t2, t2w, t1, t1w, +1, -1)
+	cmp("t2", "t2", t2, t2w, t2, t2w, 0, 0)
+	cmp("t2", "t3", t2, t2w, t3, t3w, -1, -1)
+
+	cmp("t3", "t1", t3, t3w, t1, t1w, +1, 0)
+	cmp("t3", "t2", t3, t3w, t2, t2w, +1, +1)
+	cmp("t3", "t3", t3, t3w, t3, t3w, 0, 0)
+}
+
+func TestMonotonicOverflow(t *testing.T) {
+	t1 := Now().Add(-30 * Second)
+	d := Until(t1)
+	if d < -35*Second || -30*Second < d {
+		t.Errorf("Until(Now().Add(-30s)) = %v, want roughly -30s (-35s to -30s)", d)
+	}
+
+	t1 = Now().Add(30 * Second)
+	d = Until(t1)
+	if d < 25*Second || 30*Second < d {
+		t.Errorf("Until(Now().Add(-30s)) = %v, want roughly 30s (25s to 30s)", d)
+	}
+
+	t0 := Now()
+	t1 = t0.Add(Duration(1<<63 - 1))
+	if GetMono(&t1) != 0 {
+		t.Errorf("Now().Add(maxDuration) has monotonic clock reading (%v => %v %d %d)", t0.String(), t1.String(), t0.Unix(), t1.Unix())
+	}
+	t2 := t1.Add(-Duration(1<<63 - 1))
+	d = Since(t2)
+	if d < -10*Second || 10*Second < d {
+		t.Errorf("Since(Now().Add(max).Add(-max)) = %v, want [-10s, 10s]", d)
+	}
+
+	t0 = Now()
+	t1 = t0.Add(1 * Hour)
+	Sleep(100 * Millisecond)
+	t2 = Now().Add(-5 * Second)
+	if !t1.After(t2) {
+		t.Errorf("Now().Add(1*Hour).After(Now().Add(-5*Second)) = false, want true\nt1=%v\nt2=%v", t1, t2)
+	}
+	if t2.After(t1) {
+		t.Errorf("Now().Add(-5*Second).After(Now().Add(1*Hour)) = true, want false\nt1=%v\nt2=%v", t1, t2)
+	}
+	if t1.Before(t2) {
+		t.Errorf("Now().Add(1*Hour).Before(Now().Add(-5*Second)) = true, want false\nt1=%v\nt2=%v", t1, t2)
+	}
+	if !t2.Before(t1) {
+		t.Errorf("Now().Add(-5*Second).Before(Now().Add(1*Hour)) = false, want true\nt1=%v\nt2=%v", t1, t2)
+	}
+}
+
+func TestMonotonicString(t *testing.T) {
+	t1 := Now()
+	re := regexp.MustCompile(` m=\+[0-9]+\.[0-9]{9}$`)
+	if !re.MatchString(t1.String()) {
+		t.Errorf("Now().String() = %q, want match for /%s/", t1.String(), re)
+	}
+
+	t2 := Now().Add(-5 * Hour)
+	re = regexp.MustCompile(` m=-[0-9]+\.[0-9]{9}$`)
+	if !re.MatchString(t2.String()) {
+		t.Errorf("Now().Add(-5*Hour).String() = %q, want match for /%s/", t2.String(), re)
+	}
+
+	t3 := Now().Add(1.2e18)
+	re = regexp.MustCompile(` m=\+120[0-9]{7}\.[0-9]{9}$`)
+	if !re.MatchString(t3.String()) {
+		t.Errorf("Now().Add(12e17).String() = %q, want match for /%s/", t3.String(), re)
+	}
+}
diff --git a/src/time/time.go b/src/time/time.go
index 10b3246..8a0e169 100644
--- a/src/time/time.go
+++ b/src/time/time.go
@@ -6,6 +6,74 @@
 //
 // The calendrical calculations always assume a Gregorian calendar, with
 // no leap seconds.
+//
+// Monotonic Clocks
+//
+// Operating systems provide both a “wall clock,” which is subject to
+// changes for clock synchronization, and a “monotonic clock,” which is
+// not. The general rule is that the wall clock is for telling time and
+// the monotonic clock is for measuring time. Rather than split the API,
+// in this package the Time returned by time.Now contains both a wall
+// clock reading and a monotonic clock reading; later time-telling
+// operations use the wall clock reading, but later time-measuring
+// operations, specifically comparisons and subtractions, use the
+// monotonic clock reading.
+//
+// For example, this code always computes a positive elapsed time of
+// approximately 20 milliseconds, even if the wall clock is changed during
+// the operation being timed:
+//
+//	t := time.Now()
+//	... operation that takes 20 milliseconds ...
+//	u := time.Now()
+//	elapsed := t.Sub(u)
+//
+// Other idioms, such as time.Since(start), time.Until(deadline), and
+// time.Now().Before(deadline), are similarly robust against wall clock
+// resets.
+//
+// The rest of this section gives the precise details of how operations
+// use monotonic clocks, but understanding those details is not required
+// to use this package.
+//
+// The Time returned by time.Now contains a monotonic clock reading.
+// If Time t has a monotonic clock reading, t.Add, t.Round, and
+// t.Truncate add the same duration to both the wall clock and
+// monotonic clock readings to compute the result. Similarly, t.In,
+// t.Local, and t.UTC, which are defined to change only the Time's
+// Location, pass any monotonic clock reading through unmodified.
+// Because t.AddDate(y, m, d) is a wall time computation, it always
+// strips any monotonic clock reading from its result.
+//
+// If Times t and u both contain monotonic clock readings, the operations
+// t.After(u), t.Before(u), t.Equal(u), and t.Sub(u) are carried out
+// using the monotonic clock readings alone, ignoring the wall clock
+// readings. If either t or u contains no monotonic clock reading, these
+// operations fall back to using the wall clock readings.
+//
+// Because the monotonic clock reading has no meaning outside
+// the current process, the serialized forms generated by t.GobEncode,
+// t.MarshalBinary, t.MarshalJSON, and t.MarshalText omit the monotonic
+// clock reading, and t.Format provides no format for it. Similarly, the
+// constructors time.Date, time.Parse, time.ParseInLocation, and time.Unix,
+// as well as the unmarshalers t.GobDecode, t.UnmarshalBinary.
+// t.UnmarshalJSON, and t.UnmarshalText always create times with
+// no monotonic clock reading.
+//
+// Note that the Go == operator includes the monotonic clock reading in
+// its comparison. If time values returned from time.Now and time values
+// constructed by other means (for example, by time.Parse or time.Unix)
+// are meant to compare equal when used as map keys, the times returned
+// by time.Now must have the monotonic clock reading stripped, by setting
+// t = t.AddDate(0, 0, 0). In general, prefer t.Equal(u) to t == u, since
+// t.Equal uses the most accurate comparison available and correctly
+// handles the case when only one of its arguments has a monotonic clock
+// reading.
+//
+// For debugging, the result of t.String does include the monotonic
+// clock reading if present. If t != u because of different monotonic clock readings,
+// that difference will be visible when printing t.String() and u.String().
+//
 package time
 
 import "errors"
@@ -37,15 +105,25 @@
 // without first guaranteeing that the identical Location has been set for all
 // values, which can be achieved through use of the UTC or Local method.
 //
+// In addition to the required “wall clock” reading, a Time may contain an optional
+// reading of the current process's monotonic clock, to provide additional precision
+// for comparison or subtraction.
+// See the “Monotonic Clocks” section in the package documentation for details.
+//
 type Time struct {
-	// sec gives the number of seconds elapsed since
-	// January 1, year 1 00:00:00 UTC.
-	sec int64
-
-	// nsec specifies a non-negative nanosecond
-	// offset within the second named by Seconds.
-	// It must be in the range [0, 999999999].
-	nsec int32
+	// wall and ext encode the wall time seconds, wall time nanoseconds,
+	// and optional monotonic clock reading in nanoseconds.
+	//
+	// From high to low bit position, wall encodes a 1-bit flag (hasMonotonic),
+	// a 33-bit seconds field, and a 30-bit wall time nanoseconds field.
+	// The nanoseconds field is in the range [0, 999999999].
+	// If the hasMonotonic bit is 0, then the 33-bit field must be zero
+	// and the full signed 64-bit wall seconds since Jan 1 year 1 is stored in ext.
+	// If the hasMonotonic bit is 1, then the 33-bit field holds a 33-bit
+	// unsigned wall seconds since Jan 1 year 1885, and ext holds a
+	// signed 64-bit monotonic clock reading, nanoseconds since process start.
+	wall uint64
+	ext  int64
 
 	// loc specifies the Location that should be used to
 	// determine the minute, hour, month, day, and year
@@ -55,6 +133,54 @@
 	loc *Location
 }
 
+const (
+	hasMonotonic = 1 << 63
+	maxWall      = wallToInternal + (1<<33 - 1) // year 2157
+	minWall      = wallToInternal               // year 1885
+	nsecMask     = 1<<30 - 1
+	nsecShift    = 30
+)
+
+// These helpers for manipulating the wall and monotonic clock readings
+// take pointer receivers, even when they don't modify the time,
+// to make them cheaper to call.
+
+// nsec returns the time's nanoseconds.
+func (t *Time) nsec() int32 {
+	return int32(t.wall & nsecMask)
+}
+
+// sec returns the time's seconds since Jan 1 year 1.
+func (t *Time) sec() int64 {
+	if t.wall&hasMonotonic != 0 {
+		return wallToInternal + int64(t.wall<<1>>(nsecShift+1))
+	}
+	return int64(t.ext)
+}
+
+// unixSec returns the time's seconds since Jan 1 1970 (Unix time).
+func (t *Time) unixSec() int64 { return t.sec() + internalToUnix }
+
+// addSec adds d seconds to the time.
+func (t *Time) addSec(d int64) {
+	if t.wall&hasMonotonic != 0 {
+		sec := int64(t.wall << 1 >> (nsecShift + 1))
+		dsec := sec + d
+		if 0 <= dsec && dsec <= 1<<33-1 {
+			t.wall = t.wall&nsecMask | uint64(dsec)<<nsecShift | hasMonotonic
+			return
+		}
+		// Wall second now out of range for packed field.
+		// Move to ext.
+		t.ext = t.sec()
+		t.wall &= nsecMask
+	}
+
+	// TODO: Check for overflow.
+	t.ext += d
+}
+
+// setLoc sets the location associated with the time.
 func (t *Time) setLoc(loc *Location) {
 	if loc == &utcLoc {
 		loc = nil
@@ -62,14 +188,49 @@
 	t.loc = loc
 }
 
+// setMono sets the monotonic clock reading in t.
+// If t cannot hold a monotonic clock reading,
+// because its wall time is too large,
+// setMono is a no-op.
+func (t *Time) setMono(m int64) {
+	if t.wall&hasMonotonic == 0 {
+		sec := int64(t.ext)
+		if sec < minWall || maxWall < sec {
+			return
+		}
+		t.wall |= hasMonotonic | uint64(sec-minWall)<<nsecShift
+	}
+	t.ext = m
+}
+
+// mono returns t's monotonic clock reading.
+// It returns 0 for a missing reading.
+// This function is used only for testing,
+// so it's OK that technically 0 is a valid
+// monotonic clock reading as well.
+func (t *Time) mono() int64 {
+	if t.wall&hasMonotonic == 0 {
+		return 0
+	}
+	return t.ext
+}
+
 // After reports whether the time instant t is after u.
 func (t Time) After(u Time) bool {
-	return t.sec > u.sec || t.sec == u.sec && t.nsec > u.nsec
+	if t.wall&u.wall&hasMonotonic != 0 {
+		return t.ext > u.ext
+	}
+	ts := t.sec()
+	us := u.sec()
+	return ts > us || ts == us && t.nsec() > u.nsec()
 }
 
 // Before reports whether the time instant t is before u.
 func (t Time) Before(u Time) bool {
-	return t.sec < u.sec || t.sec == u.sec && t.nsec < u.nsec
+	if t.wall&u.wall&hasMonotonic != 0 {
+		return t.ext < u.ext
+	}
+	return t.sec() < u.sec() || t.sec() == u.sec() && t.nsec() < u.nsec()
 }
 
 // Equal reports whether t and u represent the same time instant.
@@ -77,7 +238,10 @@
 // For example, 6:00 +0200 CEST and 4:00 UTC are Equal.
 // Do not use == with Time values.
 func (t Time) Equal(u Time) bool {
-	return t.sec == u.sec && t.nsec == u.nsec
+	if t.wall&u.wall&hasMonotonic != 0 {
+		return t.ext == u.ext
+	}
+	return t.sec() == u.sec() && t.nsec() == u.nsec()
 }
 
 // A Month specifies a month of the year (January = 1, ...).
@@ -245,12 +409,15 @@
 
 	unixToInternal int64 = (1969*365 + 1969/4 - 1969/100 + 1969/400) * secondsPerDay
 	internalToUnix int64 = -unixToInternal
+
+	wallToInternal int64 = (1884*365 + 1884/4 - 1884/100 + 1884/400) * secondsPerDay
+	internalToWall int64 = -wallToInternal
 )
 
 // IsZero reports whether t represents the zero time instant,
 // January 1, year 1, 00:00:00 UTC.
 func (t Time) IsZero() bool {
-	return t.sec == 0 && t.nsec == 0
+	return t.sec() == 0 && t.nsec() == 0
 }
 
 // abs returns the time t as an absolute time, adjusted by the zone offset.
@@ -261,7 +428,7 @@
 	if l == nil || l == &localLoc {
 		l = l.get()
 	}
-	sec := t.sec + internalToUnix
+	sec := t.unixSec()
 	if l != &utcLoc {
 		if l.cacheZone != nil && l.cacheStart <= sec && sec < l.cacheEnd {
 			sec += int64(l.cacheZone.offset)
@@ -281,7 +448,7 @@
 		l = l.get()
 	}
 	// Avoid function call if we hit the local time cache.
-	sec := t.sec + internalToUnix
+	sec := t.unixSec()
 	if l != &utcLoc {
 		if l.cacheZone != nil && l.cacheStart <= sec && sec < l.cacheEnd {
 			name = l.cacheZone.name
@@ -425,7 +592,7 @@
 // Nanosecond returns the nanosecond offset within the second specified by t,
 // in the range [0, 999999999].
 func (t Time) Nanosecond() int {
-	return int(t.nsec)
+	return int(t.nsec())
 }
 
 // YearDay returns the day of the year specified by t, in the range [1,365] for non-leap years,
@@ -618,16 +785,27 @@
 
 // Add returns the time t+d.
 func (t Time) Add(d Duration) Time {
-	t.sec += int64(d / 1e9)
-	nsec := t.nsec + int32(d%1e9)
+	dsec := int64(d / 1e9)
+	nsec := t.nsec() + int32(d%1e9)
 	if nsec >= 1e9 {
-		t.sec++
+		dsec++
 		nsec -= 1e9
 	} else if nsec < 0 {
-		t.sec--
+		dsec--
 		nsec += 1e9
 	}
-	t.nsec = nsec
+	t.wall = t.wall&^nsecMask | uint64(nsec) // update nsec
+	t.addSec(dsec)
+	if t.wall&hasMonotonic != 0 {
+		te := t.ext + int64(d)
+		if d < 0 && te > int64(t.ext) || d > 0 && te < int64(t.ext) {
+			// Monotonic clock reading now out of range; degrade to wall-only.
+			t.ext = t.sec()
+			t.wall &= nsecMask
+		} else {
+			t.ext = te
+		}
+	}
 	return t
 }
 
@@ -636,7 +814,19 @@
 // will be returned.
 // To compute t-d for a duration d, use t.Add(-d).
 func (t Time) Sub(u Time) Duration {
-	d := Duration(t.sec-u.sec)*Second + Duration(t.nsec-u.nsec)
+	if t.wall&u.wall&hasMonotonic != 0 {
+		te := int64(t.ext)
+		ue := int64(u.ext)
+		d := Duration(te - ue)
+		if d < 0 && te > ue {
+			return maxDuration // t - u is positive out of range
+		}
+		if d > 0 && te < ue {
+			return minDuration // t - u is negative out of range
+		}
+		return d
+	}
+	d := Duration(t.sec()-u.sec())*Second + Duration(t.nsec()-u.nsec())
 	// Check for overflow or underflow.
 	switch {
 	case u.Add(d).Equal(t):
@@ -671,7 +861,7 @@
 func (t Time) AddDate(years int, months int, days int) Time {
 	year, month, day := t.Date()
 	hour, min, sec := t.Clock()
-	return Date(year+years, month+Month(months), day+days, hour, min, sec, int(t.nsec), t.Location())
+	return Date(year+years, month+Month(months), day+days, hour, min, sec, int(t.nsec()), t.Location())
 }
 
 const (
@@ -791,12 +981,18 @@
 }
 
 // Provided by package runtime.
-func now() (sec int64, nsec int32)
+func now() (sec int64, nsec int32, mono uint64)
 
 // Now returns the current local time.
 func Now() Time {
-	sec, nsec := now()
-	return Time{sec + unixToInternal, nsec, Local}
+	sec, nsec, mono := now()
+	t := unixTime(sec, nsec)
+	t.setMono(int64(mono))
+	return t
+}
+
+func unixTime(sec int64, nsec int32) Time {
+	return Time{uint64(nsec), sec + unixToInternal, Local}
 }
 
 // UTC returns t with the location set to UTC.
@@ -834,14 +1030,14 @@
 // Zone computes the time zone in effect at time t, returning the abbreviated
 // name of the zone (such as "CET") and its offset in seconds east of UTC.
 func (t Time) Zone() (name string, offset int) {
-	name, offset, _, _, _ = t.loc.lookup(t.sec + internalToUnix)
+	name, offset, _, _, _ = t.loc.lookup(t.unixSec())
 	return
 }
 
 // Unix returns t as a Unix time, the number of seconds elapsed
 // since January 1, 1970 UTC.
 func (t Time) Unix() int64 {
-	return t.sec + internalToUnix
+	return t.unixSec()
 }
 
 // UnixNano returns t as a Unix time, the number of nanoseconds elapsed
@@ -850,7 +1046,7 @@
 // 1678 or after 2262). Note that this means the result of calling UnixNano
 // on the zero Time is undefined.
 func (t Time) UnixNano() int64 {
-	return (t.sec+internalToUnix)*1e9 + int64(t.nsec)
+	return (t.unixSec())*1e9 + int64(t.nsec())
 }
 
 const timeBinaryVersion byte = 1
@@ -873,20 +1069,22 @@
 		offsetMin = int16(offset)
 	}
 
+	sec := t.sec()
+	nsec := t.nsec()
 	enc := []byte{
 		timeBinaryVersion, // byte 0 : version
-		byte(t.sec >> 56), // bytes 1-8: seconds
-		byte(t.sec >> 48),
-		byte(t.sec >> 40),
-		byte(t.sec >> 32),
-		byte(t.sec >> 24),
-		byte(t.sec >> 16),
-		byte(t.sec >> 8),
-		byte(t.sec),
-		byte(t.nsec >> 24), // bytes 9-12: nanoseconds
-		byte(t.nsec >> 16),
-		byte(t.nsec >> 8),
-		byte(t.nsec),
+		byte(sec >> 56),   // bytes 1-8: seconds
+		byte(sec >> 48),
+		byte(sec >> 40),
+		byte(sec >> 32),
+		byte(sec >> 24),
+		byte(sec >> 16),
+		byte(sec >> 8),
+		byte(sec),
+		byte(nsec >> 24), // bytes 9-12: nanoseconds
+		byte(nsec >> 16),
+		byte(nsec >> 8),
+		byte(nsec),
 		byte(offsetMin >> 8), // bytes 13-14: zone offset in minutes
 		byte(offsetMin),
 	}
@@ -910,18 +1108,22 @@
 	}
 
 	buf = buf[1:]
-	t.sec = int64(buf[7]) | int64(buf[6])<<8 | int64(buf[5])<<16 | int64(buf[4])<<24 |
+	sec := int64(buf[7]) | int64(buf[6])<<8 | int64(buf[5])<<16 | int64(buf[4])<<24 |
 		int64(buf[3])<<32 | int64(buf[2])<<40 | int64(buf[1])<<48 | int64(buf[0])<<56
 
 	buf = buf[8:]
-	t.nsec = int32(buf[3]) | int32(buf[2])<<8 | int32(buf[1])<<16 | int32(buf[0])<<24
+	nsec := int32(buf[3]) | int32(buf[2])<<8 | int32(buf[1])<<16 | int32(buf[0])<<24
 
 	buf = buf[4:]
 	offset := int(int16(buf[1])|int16(buf[0])<<8) * 60
 
+	*t = Time{}
+	t.wall = uint64(nsec)
+	t.ext = sec
+
 	if offset == -1*60 {
 		t.setLoc(&utcLoc)
-	} else if _, localoff, _, _, _ := Local.lookup(t.sec + internalToUnix); offset == localoff {
+	} else if _, localoff, _, _, _ := Local.lookup(t.unixSec()); offset == localoff {
 		t.setLoc(Local)
 	} else {
 		t.setLoc(FixedZone("", offset))
@@ -1008,7 +1210,7 @@
 			sec--
 		}
 	}
-	return Time{sec + unixToInternal, int32(nsec), Local}
+	return unixTime(sec, int32(nsec))
 }
 
 func isLeap(year int) bool {
@@ -1117,7 +1319,7 @@
 		unix -= int64(offset)
 	}
 
-	t := Time{unix + unixToInternal, int32(nsec), nil}
+	t := unixTime(unix, int32(nsec))
 	t.setLoc(loc)
 	return t
 }
@@ -1161,15 +1363,16 @@
 // but it's still here in case we change our minds.
 func div(t Time, d Duration) (qmod2 int, r Duration) {
 	neg := false
-	nsec := t.nsec
-	if t.sec < 0 {
+	nsec := t.nsec()
+	sec := t.sec()
+	if sec < 0 {
 		// Operate on absolute value.
 		neg = true
-		t.sec = -t.sec
+		sec = -sec
 		nsec = -nsec
 		if nsec < 0 {
 			nsec += 1e9
-			t.sec-- // t.sec >= 1 before the -- so safe
+			sec-- // sec >= 1 before the -- so safe
 		}
 	}
 
@@ -1182,8 +1385,8 @@
 	// Special case: d is a multiple of 1 second.
 	case d%Second == 0:
 		d1 := int64(d / Second)
-		qmod2 = int(t.sec/d1) & 1
-		r = Duration(t.sec%d1)*Second + Duration(nsec)
+		qmod2 = int(sec/d1) & 1
+		r = Duration(sec%d1)*Second + Duration(nsec)
 
 	// General case.
 	// This could be faster if more cleverness were applied,
@@ -1191,7 +1394,7 @@
 	// No one will care about these cases.
 	default:
 		// Compute nanoseconds as 128-bit number.
-		sec := uint64(t.sec)
+		sec := uint64(sec)
 		tmp := (sec >> 32) * 1e9
 		u1 := tmp >> 32
 		u0 := tmp << 32
diff --git a/src/time/zoneinfo_plan9.go b/src/time/zoneinfo_plan9.go
index 0694f0a..26637a1 100644
--- a/src/time/zoneinfo_plan9.go
+++ b/src/time/zoneinfo_plan9.go
@@ -95,7 +95,7 @@
 
 	// Fill in the cache with information about right now,
 	// since that will be the most common lookup.
-	sec, _ := now()
+	sec, _, _ := now()
 	for i := range tx {
 		if tx[i].when <= sec && (i+1 == len(tx) || sec < tx[i+1].when) {
 			l.cacheStart = tx[i].when
diff --git a/src/time/zoneinfo_read.go b/src/time/zoneinfo_read.go
index 19cd40d..1b3356e 100644
--- a/src/time/zoneinfo_read.go
+++ b/src/time/zoneinfo_read.go
@@ -188,7 +188,7 @@
 
 	// Fill in the cache with information about right now,
 	// since that will be the most common lookup.
-	sec, _ := now()
+	sec, _, _ := now()
 	for i := range tx {
 		if tx[i].when <= sec && (i+1 == len(tx) || sec < tx[i+1].when) {
 			l.cacheStart = tx[i].when
diff --git a/src/time/zoneinfo_windows.go b/src/time/zoneinfo_windows.go
index a6e227b..c201f4b 100644
--- a/src/time/zoneinfo_windows.go
+++ b/src/time/zoneinfo_windows.go
@@ -132,7 +132,7 @@
 			day -= 7
 		}
 	}
-	return t.sec + int64(day-1)*secondsPerDay + internalToUnix
+	return t.sec() + int64(day-1)*secondsPerDay + internalToUnix
 }
 
 func initLocalFromTZI(i *syscall.Timezoneinformation) {