fix bug with year roll over

internal/task/date.go

@@ -206,11 +206,17 @@ func (d Date) Add(days int) Date {
 }
 
 func (d Date) AddMonths(addMonths int) Date {
-	year, month, day := d.t.Date()
+	year, mmonth, day := d.t.Date()
-	addYears := int((int(month) + addMonths) / 12)
+	month := int(mmonth)
-	newMonth := (int(month) + addMonths) % 12
+	for m := 1; m <= addMonths; m++ {
+		month += 1
+		if month == 12 {
+			year += 1
+			month = 1
+		}
+	}
 
-	return NewDate(year+addYears, newMonth, day)
+	return NewDate(year, month, day)
 }
 
 func (d Date) Equal(ud Date) bool {

internal/task/recur_test.go

@@ -367,4 +367,12 @@ func TestEveryNMonths(t *testing.T) {
 		}
 		test.Equals(t, false, recur.RecursOn(task.NewDate(2021, 3, 9)))
 	})
+
+	t.Run("bug", func(t *testing.T) {
+		recur := task.EveryNMonths{
+			Start: task.NewDate(2021, 3, 1),
+			N:     1,
+		}
+		test.Equals(t, false, recur.RecursOn(task.NewDate(2021, 11, 3)))
+	})
 }