Fix diff in months counting months twice

carbon.go

@@ -1187,7 +1187,7 @@ func (c *Carbon) DiffInMonths(carb *Carbon, abs bool) int64 {
 	if (diffHr - hrLastMonth) >= 0 {
 		var m int64
 		if c.Year() < carb.Year() {
-			m = (int64(monthsPerYear) - int64(c.In(time.UTC).Month())) + (int64(carb.In(time.UTC).Month()) - 1)
+			m = int64(monthsPerYear) - int64(c.In(time.UTC).Month()) + int64(carb.In(time.UTC).Month()) - 1
 			totalHr := int64(c.DaysInMonth() * hoursPerDay)
 			cHr := c.StartOfMonth().DiffInHours(c, abs)
 			remainHr := totalHr - cHr
@@ -1210,7 +1210,8 @@ func (c *Carbon) DiffInMonths(carb *Carbon, abs bool) int64 {
 
 		diffYr := c.Year() - carb.Year()
 		if math.Abs(float64(diffYr)) > 1 {
-			diff := c.DiffInYears(carb, abs)*monthsPerYear + m
+			dateWithoutMonths := c.AddMonths(int(m))
+			diff := dateWithoutMonths.DiffInYears(carb, abs)*monthsPerYear + m
 
 			return absValue(abs, diff)
 		}

carbon_test.go

@@ -1993,6 +1993,13 @@ func TestDiffInMonthsSameMonth(t *testing.T) {
 	assert.EqualValues(t, 0, t1.DiffInMonths(t2, true))
 }
 
+func TestDiffInMonthsDifferentYears(t *testing.T) {
+	t1, _ := Create(2018, time.May, 1, 0, 0, 0, 0, "UTC")
+	t2, _ := Create(2020, time.June, 1, 0, 0, 0, 0, "UTC")
+
+	assert.EqualValues(t, 25, t1.DiffInMonths(t2, true))
+}
+
 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")