Add IsDST

example_test.go

@@ -120,3 +120,19 @@ func ExampleTimezone_GetTimezoneAbbreviation_dst() {
 	// Output:
 	// BST
 }
+
+func ExampleTimezone_IsDST() {
+	tz := timezone.New()
+
+	loc, _ := time.LoadLocation("America/New_York")
+	_time := time.Date(2021, 7, 1, 0, 0, 0, 0, loc)
+	isDST := tz.IsDST(_time)
+
+	_time = time.Date(2021, 1, 1, 0, 0, 0, 0, loc)
+	isNotDST := tz.IsDST(_time)
+	fmt.Println(isDST)
+	fmt.Println(isNotDST)
+	// Output:
+	// true
+	// false
+}

timezone.go

@@ -174,3 +174,28 @@ func (tz *Timezone) GetTimezoneAbbreviation(timezone string, dst ...bool) (strin
 
 	return tzinfo.ShortDaylight(), nil
 }
+
+// IsDST returns whether a given time is daylight saving time or not.
+func (tz *Timezone) IsDST(t time.Time) bool {
+	t1 := time.Date(t.Year(), time.January, 1, 0, 0, 0, 0, t.Location())
+	t2 := time.Date(t.Year(), time.July, 1, 0, 0, 0, 0, t.Location())
+
+	_, tOffset := t.Zone()
+	_, t1Offset := t1.Zone()
+	_, t2Offset := t2.Zone()
+
+	var dstOffset int
+	if t1Offset > t2Offset {
+		dstOffset = t1Offset
+	} else if t1Offset < t2Offset {
+		dstOffset = t2Offset
+	} else {
+		return false
+	}
+
+	if dstOffset == tOffset {
+		return true
+	}
+
+	return false
+}

timezone_test.go

@@ -216,3 +216,49 @@ func TestGetTimezoneAbbreviation(t *testing.T) {
 		t.Fatal("Invalid timezone")
 	}
 }
+
+func TestIsDST(t *testing.T) {
+	t.Parallel()
+
+	tz := New()
+
+	timezone := "America/New_York"
+	expect := false
+	loc, err := time.LoadLocation(timezone)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	tNotDST := time.Date(2021, 1, 1, 1, 0, 0, 0, loc)
+	isDST := tz.IsDST(tNotDST)
+	if isDST {
+		t.Fatalf(`expected: %v, actual: %v`, expect, isDST)
+	}
+
+	expect = true
+
+	tDST := time.Date(2021, 7, 1, 1, 0, 0, 0, loc)
+	isDST = tz.IsDST(tDST)
+	if !isDST {
+		t.Fatalf(`expected: %v, actual: %v`, expect, isDST)
+	}
+
+	timezone = "UTC"
+	expect = false
+	loc, err = time.LoadLocation(timezone)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	tNotDST = time.Date(2021, 1, 1, 1, 0, 0, 0, loc)
+	isDST = tz.IsDST(tNotDST)
+	if isDST {
+		t.Fatalf(`expected: %v, actual: %v`, expect, isDST)
+	}
+
+	tNotDST = time.Date(2021, 7, 1, 1, 0, 0, 0, loc)
+	isDST = tz.IsDST(tNotDST)
+	if isDST {
+		t.Fatalf(`expected: %v, actual: %v`, expect, isDST)
+	}
+}