[RFC] Fix incorrect diff in months (#74)

* Fix incorrect diff in months

* Improve naming of test

carbon.go

@@ -1202,9 +1202,9 @@ func calculateDiffInMonths(c, carb *Carbon, abs bool) int64 {
 
 	if c.Month() != carb.Month() && c.Year() == carb.Year() {
 		diffInMonths := int64(carb.Month() - c.Month())
-		remainingTime := int(carb.DiffInHours(c, true))
+		remainingTime, totalHours := calculateDiffInTimeAndHours(c, carb)
 
-		if remainingTime < c.DaysInMonth()*hoursPerDay {
+		if remainingTime < totalHours {
 			return 0
 		}
 
@@ -1838,6 +1838,14 @@ func (c *Carbon) SetLocale(l string) error {
 	return nil
 }
 
+func calculateDiffInTimeAndHours(c, carb *Carbon) (int, int) {
+	if carb.Timestamp() < c.Timestamp() {
+		return int(c.DiffInHours(carb, true)), carb.DaysInMonth() * hoursPerDay
+	}
+
+	return int(carb.DiffInHours(c, true)), c.DaysInMonth() * hoursPerDay
+}
+
 /* TODO
 // String Formatting
 

carbon_test.go

@@ -2071,6 +2071,17 @@ func TestDiffInMonthsEnsureIsTruncated(t *testing.T) {
 	assert.EqualValues(t, 1, t1.DiffInMonths(t2, true))
 }
 
+func TestDiffInMonthsFor30DaysMonths(t *testing.T) {
+	d1, _ := Create(2020, time.November, 1, 0, 0, 0, 0, time.UTC.String())
+	d2, _ := Create(2020, time.December, 1, 0, 0, 0, 0, time.UTC.String())
+
+	months := d1.DiffInMonths(d2, true)
+	assert.EqualValues(t, 1, months)
+
+	monthsReverse := d2.DiffInMonths(d1, true)
+	assert.EqualValues(t, 1, monthsReverse)
+}
+
 func TestDiffInString(t *testing.T) {
 	t1, _ := Create(2016, time.August, 10, 10, 0, 0, 0, "UTC")
 	t2, _ := Create(2016, time.August, 1, 23, 0, 0, 0, "UTC")