pkg/time: use UTC time zone for Unix and Parse

Change-Id: Ia38856c40bfca70620c7783d41ea3936a020b745
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/2726
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/pkg/time/time.go b/pkg/time/time.go
index eadcaa8..d93ac5e 100644
--- a/pkg/time/time.go
+++ b/pkg/time/time.go
@@ -189,7 +189,7 @@
 	if err != nil {
 		return "", err
 	}
-	return t.Format(time.RFC3339Nano), nil
+	return t.UTC().Format(time.RFC3339Nano), nil
 }
 
 // Unix returns the local Time corresponding to the given Unix time,
@@ -199,5 +199,5 @@
 // value is 1<<63-1 (the largest int64 value).
 func Unix(sec int64, nsec int64) string {
 	t := time.Unix(sec, nsec)
-	return t.Format(time.RFC3339Nano)
+	return t.UTC().Format(time.RFC3339Nano)
 }
diff --git a/pkg/time/time_test.go b/pkg/time/time_test.go
index 0386c66..7646ee6 100644
--- a/pkg/time/time_test.go
+++ b/pkg/time/time_test.go
@@ -102,8 +102,8 @@
 		nano int64
 		want string
 	}{
-		{0, 0, "1970-01-01T01:00:00+01:00"},
-		{1500000000, 123456, "2017-07-14T04:40:00.000123456+02:00"},
+		{0, 0, "1970-01-01T00:00:00Z"},
+		{1500000000, 123456, "2017-07-14T02:40:00.000123456Z"},
 	}
 
 	for _, tc := range valid {