unix: add unix.TimeToPtpClockTime on Linux

Add time conversion helpers for PtpClockTime similar to those
existing for Timespec and Timeval.

Allows to use the maths of the standard time package while avoiding
the boilerplate:
write "ptpt.Time()" instead of "time.Unix(ptpt.Sec, int64(ptpt.Nsec))",
and "unix.TimeToPtpClockTime(t)" instead of
"unix.PtpClockTime{Sec: t.Unix(), Nsec: uint32(t.Nanosecond())}".

unix/timestruct_linux.go

@@ -0,0 +1,26 @@
+// Copyright 2024 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.
+
+//go:build linux
+
+package unix
+
+import "time"
+
+// TimeToPtpClockTime returns t as PtpClockTime
+func TimeToPtpClockTime(t time.Time) PtpClockTime {
+	sec := t.Unix()
+	nsec := uint32(t.Nanosecond())
+	return PtpClockTime{Sec: sec, Nsec: nsec}
+}
+
+// Time returns PTPClockTime as time.Time
+func (t *PtpClockTime) Time() time.Time {
+	return time.Unix(t.Sec, int64(t.Nsec))
+}
+
+// Unix returns the time stored in t as seconds plus nanoseconds.
+func (t *PtpClockTime) Unix() (sec int64, nsec int64) {
+	return t.Sec, int64(t.Nsec)
+}

unix/timestruct_linux_test.go

@@ -0,0 +1,42 @@
+// Copyright 2024 The Go Authors. All right reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build linux
+
+package unix_test
+
+import (
+	"testing"
+	"time"
+
+	"golang.org/x/sys/unix"
+)
+
+func TestTimeToPtpClockTime(t *testing.T) {
+	testcases := []struct {
+		time  time.Time
+	}{
+		{time.Unix(0, 0)},
+		{time.Date(2009, time.November, 10, 23, 0, 0, 0, time.UTC)},
+		{time.Date(2262, time.December, 31, 23, 0, 0, 0, time.UTC)},
+		{time.Unix(0x7FFFFFFF, 0)},
+		{time.Unix(0x80000000, 0)},
+		{time.Unix(0x7FFFFFFF, 1000000000)},
+		{time.Unix(0x7FFFFFFF, 999999999)},
+		{time.Unix(-0x80000000, 0)},
+		{time.Unix(-0x80000001, 0)},
+		{time.Date(2038, time.January, 19, 3, 14, 7, 0, time.UTC)},
+		{time.Date(2038, time.January, 19, 3, 14, 8, 0, time.UTC)},
+		{time.Date(1901, time.December, 13, 20, 45, 52, 0, time.UTC)},
+		{time.Date(1901, time.December, 13, 20, 45, 51, 0, time.UTC)},
+	}
+
+	for _, tc := range testcases {
+		ts := unix.TimeToPtpClockTime(tc.time)
+		tstime := time.Unix(int64(ts.Sec), int64(ts.Nsec))
+		if !tstime.Equal(tc.time) {
+			t.Errorf("TimeToPtpClockTime(%v) is the time %v", tc.time, tstime)
+		}
+	}
+}