/// <summary> /// Returns the next month day after the given DateTime. /// </summary> /// <param name="T">Source DateTime</param> /// <param name="D">Target month day</param> /// <returns>DateTime</returns> public DateTime NextHigherMonthDay(DateTime T, int D) { return ((T.Day >= D) ? T.AddMonths(1) : T).AddDays(D-T.Day); } // With a minor change, you can create related functions like // the one below. I just changed the ">=" to ">" ... public DateTime NextHigherOrEqualMonthDay(DateTime T, int D) { return ((T.Day > D) ? T.AddMonths(1) : T).AddDays(D - T.Day); } //Example: // Testdate (August 10, 2007): DateTime TestDate = new DateTime(2007, 8, 10, 20, 15, 0); // Test 1: DateTime NewDate1 = NextHigherMonthDay(TestDate, 10); // The will be: September 10, 2007 because it is // the next 10th day after the given date // Test 2: DateTime NewDate2 = NextHigherMonthDay(TestDate, 11); // The will be: August 11, 2007 because it is // the next 11th day after the given date // Test 3: DateTime NewDate3 = NextHigherMonthDay(TestDate, 3); // The will be: September 3, 2007 because it is // the next 3rd day after the given date