[BEAM-13988] Update mtime to use time.UnixMilli() calls (#17578)

sdks/go/pkg/beam/core/graph/mtime/time.go

@@ -65,8 +65,7 @@ func FromDuration(d time.Duration) Time {
 
 // FromTime returns a milli-second precision timestamp from a time.Time.
 func FromTime(t time.Time) Time {
-	// TODO(BEAM-13988): Replace t.UnixNano() with t.UnixMilli() for Go 1.17 or higher.
-	return Normalize(Time(t.UnixNano() / 1e6))
+	return Normalize(Time(t.UnixMilli()))
 }
 
 // Milliseconds returns the number of milli-seconds since the Unix epoch.
@@ -76,8 +75,7 @@ func (t Time) Milliseconds() int64 {
 
 // ToTime returns the Time represented as a time.Time
 func (t Time) ToTime() time.Time {
-	// TODO(BEAM-13988): Replace with time.UnixMilli(int64(t)).UTC() for Go 1.17 or higher.
-	return time.Unix(0, int64(t)*1e6).UTC()
+	return time.UnixMilli(int64(t)).UTC()
 }
 
 // Add returns the time plus the duration. Input Durations of less than one

sdks/go/pkg/beam/core/graph/mtime/time_test.go

@@ -244,3 +244,57 @@ func TestToTime(t *testing.T) {
 		})
 	}
 }
+
+func TestMilliseconds(t *testing.T) {
+	tests := []struct {
+		name        string
+		inputMillis int64
+	}{
+		{
+			"Zero",
+			int64(0),
+		},
+		{
+			"End of Global Window",
+			EndOfGlobalWindowTime.Milliseconds(),
+		},
+		{
+			"some number",
+			int64(1000),
+		},
+	}
+	for _, test := range tests {
+		milliTime := FromMilliseconds(test.inputMillis)
+		outputMillis := milliTime.Milliseconds()
+		if got, want := outputMillis, test.inputMillis; got != want {
+			t.Errorf("got %v milliseconds, want %v milliseconds", got, want)
+		}
+	}
+}
+
+func TestFromDuration(t *testing.T) {
+	tests := []struct {
+		name string
+		dur  time.Duration
+	}{
+		{
+			"zero",
+			0 * time.Millisecond,
+		},
+		{
+			"End of Global Window",
+			time.Duration(EndOfGlobalWindowTime),
+		},
+		{
+			"day",
+			24 * time.Hour,
+		},
+	}
+	for _, test := range tests {
+		durTime := FromDuration(test.dur)
+		timeMillis := durTime.Milliseconds()
+		if got, want := timeMillis, test.dur.Milliseconds(); got != want {
+			t.Errorf("got %v milliseconds, want %v milliseconds", got, want)
+		}
+	}
+}