impl/memory: Fix time serialization encoding.

service/datastore/serialize/serialize.go

Index: service/datastore/serialize/serialize.go
diff --git a/service/datastore/serialize/serialize.go b/service/datastore/serialize/serialize.go
index 736010820f222499212728ca30154029a905cc4f..1f44896b3ee121da69b69d1867d72aac50ccaee6 100644
--- a/service/datastore/serialize/serialize.go
+++ b/service/datastore/serialize/serialize.go
@@ -189,16 +189,35 @@ func ReadGeoPoint(buf Buffer) (gp ds.GeoPoint, err error) {
 	return
 }
 
-// WriteTime writes a time.Time in a byte-sortable way.
+// TimeToInt converts a time value to a datastore-appropraite integer value.
 //
 // This method truncates the time to microseconds and drops the timezone,
 // because that's the (undocumented) way that the appengine SDK does it.
+func TimeToInt(t time.Time) int64 {
+	t = t.Round(time.Microsecond)
+	return t.Unix()*1e6 + int64(t.Nanosecond()/1e3)
+}
+
+// IntToTime converts a time integer produced by TimeToInt to a time.Time
+// instance.
+func IntToTime(v int64) time.Time {
+	t := time.Unix(v/1e6, (v%1e6)*1e3)
+	if t.IsZero() {
+		return time.Time{}
+	}
+	return t.UTC()
+}
+
+// WriteTime writes a time.Time in a byte-sortable way.
+//
+// This method uses TimeToInt to convert the time to an integer value. This
+// truncates the time to microseconds and drops the timezone.
 func WriteTime(buf Buffer, t time.Time) error {
 	name, off := t.Zone()
 	if name != "UTC" || off != 0 {
 		panic(fmt.Errorf("helper: UTC OR DEATH: %s", t))
 	}
-	_, err := cmpbin.WriteInt(buf, t.Unix()*1e6+int64(t.Nanosecond()/1e3))
+	_, err := cmpbin.WriteInt(buf, TimeToInt(t))
 	return err
 }
 
@@ -208,11 +227,7 @@ func ReadTime(buf Buffer) (time.Time, error) {
 	if err != nil {
 		return time.Time{}, err
 	}
-	t := time.Unix(v/1e6, (v%1e6)*1e3)
-	if t.IsZero() {
-		return time.Time{}, nil
-	}
-	return t.UTC(), nil
+	return IntToTime(v), nil
 }
 
 // WriteProperty writes a Property to the buffer. `context` behaves the same