сделал Diff_dates()

calendar/calendar.go

@@ -97,3 +97,51 @@ func (d *HoursMinutesSeconds) UnmarshalString(str string) error {
 
 	return nil
 }
+
+// Diff_dates - вычисляет разницу между двумя датами
+func Diff_dates(a, b time.Time) (year, month, day, hour, min, sec int) {
+	if a.Location() != b.Location() {
+		b = b.In(a.Location())
+	}
+	if a.After(b) {
+		a, b = b, a
+	}
+	y1, M1, d1 := a.Date()
+	y2, M2, d2 := b.Date()
+
+	h1, m1, s1 := a.Clock()
+	h2, m2, s2 := b.Clock()
+
+	year = int(y2 - y1)
+	month = int(M2 - M1)
+	day = int(d2 - d1)
+	hour = int(h2 - h1)
+	min = int(m2 - m1)
+	sec = int(s2 - s1)
+
+	// Normalize negative values
+	if sec < 0 {
+		sec += 60
+		min--
+	}
+	if min < 0 {
+		min += 60
+		hour--
+	}
+	if hour < 0 {
+		hour += 24
+		day--
+	}
+	if day < 0 {
+		// days in month:
+		t := time.Date(y1, M1, 32, 0, 0, 0, 0, time.UTC)
+		day += 32 - t.Day()
+		month--
+	}
+	if month < 0 {
+		month += 12
+		year--
+	}
+
+	return
+}

calendar/calendar_test.go

@@ -32,3 +32,13 @@ func TestHoursMinutesSeconds_UnmarshalByte(t *testing.T) {
 		t.Error("TestHoursMinutesSeconds_UnmarshalByte error")
 	}
 }
+
+func TestDiff_dates(t *testing.T) {
+
+	Date1 := time.Date(2023, 03, 19, 0, 0, 0, 0, constants.Loc)
+	Date2 := time.Date(2023, 03, 20, 0, 0, 0, 0, constants.Loc)
+	_, _, days, _, _, _ := Diff_dates(Date1, Date2)
+	if days != 1 {
+		t.Error("TestDiff_dates error")
+	}
+}