fix overflow bug with adding months

internal/task/date.go

@@ -53,7 +53,7 @@ type Date struct {
 
 func NewDate(year, month, day int) Date {
 
-	if year == 0 && month == 0 && day == 0 {
+	if year == 0 || month == 0 || month > 12 || day == 0 {
 		return Date{}
 	}
 
@@ -209,6 +209,14 @@ func (d Date) Add(days int) Date {
 	return NewDate(year, int(month), day+days)
 }
 
+func (d Date) AddMonths(addMonths int) Date {
+	year, month, day := d.t.Date()
+	addYears := int((int(month) + addMonths) / 12)
+	newMonth := (int(month) + addMonths) % 12
+
+	return NewDate(year+addYears, newMonth, day)
+}
+
 func (d Date) Equal(ud Date) bool {
 	return d.t.Equal(ud.Time())
 }

internal/task/recur.go

@@ -268,8 +268,7 @@ func (enm EveryNMonths) RecursOn(date Date) bool {
 		if tDate.After(date) {
 			return false
 		}
-		tYear, tMonth, tDay := tDate.Time().Date()
-		tDate = NewDate(tYear, int(tMonth)+enm.N, tDay)
+		tDate = tDate.AddMonths(enm.N)
 	}
 
 }

internal/task/recur_test.go

@@ -359,4 +359,12 @@ func TestEveryNMonths(t *testing.T) {
 			})
 		}
 	})
+
+	t.Run("recurs every year", func(t *testing.T) {
+		recur := task.EveryNMonths{
+			Start: task.NewDate(2021, 3, 1),
+			N:     12,
+		}
+		test.Equals(t, false, recur.RecursOn(task.NewDate(2021, 3, 9)))
+	})
 }