first day

first day of month

DateTime firstday(DateTime date)

DateTime date = new DateTime(2016, 12, 3);
DateTime firstDayOfMonth = firstday(date);       // 2016-12-01

first day of year

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

DateTime firstDayOfYear = firstday(2016)
// 2016-12-01