diff --git a/.travis.yml b/.travis.yml
index 1f58dc1..667a919 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -3,6 +3,8 @@ language: go
 go:
 - 1.11.x
 - 1.12.x
+- 1.14.x
+- 1.15.x
 
 script:
   - go fmt $(go list ./... | grep -v /vendor/)
diff --git a/gps.go b/gps.go
index c243cfd..0ca1402 100644
--- a/gps.go
+++ b/gps.go
@@ -28,10 +28,7 @@ var leaps []time.Duration
 
 // Called when package is imported.
 func init() {
-	if len(leaps) != 0 {
-		// already initialized
-		return
-	}
+	leaps = make([]time.Duration, 0)
 	previous := 0
 	for i, yearLeaps := range leapSequence {
 		year := leapSecondOrigin + i
diff --git a/gps_time.go b/gps_time.go
index cab706b..d73aa79 100644
--- a/gps_time.go
+++ b/gps_time.go
@@ -21,3 +21,23 @@ func (t GpsTime) Gps() time.Duration {
 func (t GpsTime) String() string {
 	return time.Time(t).String()
 }
+
+// ToUTC returns t as a time.Time in UTC.
+func (t GpsTime) ToUTC() time.Time {
+	return toUtcTime(t.Gps())
+}
+
+// Add returns the time t+d.
+func (t GpsTime) Add(d time.Duration) GpsTime {
+	return Gps(t.Gps() + d)
+}
+
+// Sub returns the duration t-u.
+func (t GpsTime) Sub(u GpsTime) time.Duration {
+	return t.Gps() - u.Gps()
+}
+
+// Equal reports whether t and u represent the same time instant.
+func (t GpsTime) Equal(u GpsTime) bool {
+	return t.Gps() == u.Gps()
+}
diff --git a/gps_time_test.go b/gps_time_test.go
index 4d91091..cd2128c 100644
--- a/gps_time_test.go
+++ b/gps_time_test.go
@@ -63,3 +63,84 @@ func TestString(t *testing.T) {
 		t.Errorf("String() = %v, want %v", got, want)
 	}
 }
+
+func TestGpsTime_ToUTC(t *testing.T) {
+	tests := []struct {
+		name string
+		t    GpsTime
+		want time.Time
+	}{
+		{"GPS Datum", GpsTime(gpsDatum), gpsDatum},
+		{"Inside leap table", GpsTime(time.Date(2010, time.January, 28, 16, 36, 24, 0, time.UTC)), time.Date(2010, time.January, 28, 16, 36, 24, 0, time.UTC)},
+		{"Before leap table", GpsTime(time.Date(1970, time.January, 10, 0, 0, 0, 0, time.UTC)), time.Date(1970, time.January, 10, 0, 0, 0, 0, time.UTC)},
+		{"After leap table", GpsTime(time.Date(2025, time.July, 14, 0, 0, 0, 0, time.UTC)), time.Date(2025, time.July, 14, 0, 0, 0, 0, time.UTC)},
+		{"Before leap", GpsTime(time.Date(2012, time.June, 30, 23, 59, 59, 0, time.UTC)), time.Date(2012, time.June, 30, 23, 59, 59, 0, time.UTC)},
+		{"After leap", GpsTime(time.Date(2012, time.July, 1, 0, 0, 0, 0, time.UTC)), time.Date(2012, time.July, 1, 0, 0, 0, 0, time.UTC)},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := tt.t.ToUTC(); got != tt.want {
+				t.Errorf("GpsTime.ToUTC() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}
+
+func TestGpsTime_Add(t *testing.T) {
+	tests := []struct {
+		name string
+		t    GpsTime
+		d    time.Duration
+		want GpsTime
+	}{
+		{"GPS Datum", GpsTime(gpsDatum), 5 * time.Second, GpsTime(gpsDatum.Add(5 * time.Second))},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := tt.t.Add(tt.d); got != tt.want {
+				t.Errorf("GpsTime.Add() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}
+
+func TestGpsTime_Sub(t *testing.T) {
+	tests := []struct {
+		name string
+		t    GpsTime
+		u    GpsTime
+		want time.Duration
+	}{
+		{"GPS Datum", GpsTime(gpsDatum), GpsTime(gpsDatum), 0},
+		{"Inside leap table", GpsTime(time.Date(2010, time.January, 28, 16, 36, 24, 0, time.UTC)), GpsTime(time.Date(2010, time.January, 28, 16, 36, 24, 0, time.UTC)), 0},
+		{"Inside leap table vs GPS Datum", GpsTime(time.Date(2010, time.January, 28, 16, 36, 24, 0, time.UTC)), GpsTime(gpsDatum), 948731799 * time.Second},
+		{"GPS Datum vs Inside leap table", GpsTime(gpsDatum), GpsTime(time.Date(2010, time.January, 28, 16, 36, 24, 0, time.UTC)), -948731799 * time.Second},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := tt.t.Sub(tt.u); got != tt.want {
+				t.Errorf("GpsTime.Sub() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}
+
+func TestGpsTime_Equal(t *testing.T) {
+	tests := []struct {
+		name string
+		t    GpsTime
+		u    GpsTime
+		want bool
+	}{
+		{"GPS Datum", GpsTime(gpsDatum), GpsTime(gpsDatum), true},
+		{"Inside leap table", GpsTime(time.Date(2010, time.January, 28, 16, 36, 24, 0, time.UTC)), GpsTime(time.Date(2010, time.January, 28, 16, 36, 24, 0, time.UTC)), true},
+		{"GPS Datum vs Inside leap table", GpsTime(gpsDatum), GpsTime(time.Date(2010, time.January, 28, 16, 36, 24, 0, time.UTC)), false},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := tt.t.Equal(tt.u); got != tt.want {
+				t.Errorf("GpsTime.Equal() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}