last day

last day of month

DateTime lastday(DateTime date)

DateTime date = new DateTime(2016, 12, 3);
DateTime lastDayOfMonth = lastday(date);       // 2016-12-31

last day of year

DateTime lastday(int year, DateTimeKind kind = DateTimeKind.Unspecified)

DateTime endOfYear = lastday(2016)
// 2016-12-31