Add ulid.Timestamp method (#107)

ulid.go

@@ -411,6 +411,11 @@ func (id ULID) Time() uint64 {
 		uint64(id[1])<<32 | uint64(id[0])<<40
 }
 
+// Timestamp returns the time encoded in the ULID as a time.Time.
+func (id ULID) Timestamp() time.Time {
+	return Time(id.Time())
+}
+
 // maxTime is the maximum Unix time in milliseconds that can be
 // represented in a ULID.
 var maxTime = ULID{0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF}.Time()

ulid_test.go

@@ -393,7 +393,6 @@ func TestTime(t *testing.T) {
 		t.Errorf("difference between original and recovered time (%d) greater"+
 			"than a millisecond", diff)
 	}
-
 }
 
 func TestTimestampRoundTrips(t *testing.T) {
@@ -434,6 +433,27 @@ func TestULIDTime(t *testing.T) {
 	}
 }
 
+func TestULIDTimestamp(t *testing.T) {
+	t.Parallel()
+
+	{
+		id := ulid.Make()
+		ts := id.Timestamp()
+		tt := ulid.Time(id.Time())
+		if ts != tt {
+			t.Errorf("id.Timestamp() %s != ulid.Time(id.Time()) %s", ts, tt)
+		}
+	}
+
+	{
+		now := time.Now()
+		id := ulid.MustNew(ulid.Timestamp(now), ulid.DefaultEntropy())
+		if want, have := now.Truncate(time.Millisecond), id.Timestamp(); want != have {
+			t.Errorf("Timestamp: want %v, have %v", want, have)
+		}
+	}
+}
+
 func TestEntropy(t *testing.T) {
 	t.Parallel()