firstDayOfMonth static method
Returns DateTime that represents a beginning
of the first day of the month containing date.
Example: (2020, 4, 9, 15, 16) -> (2020, 4, 1, 0, 0, 0, 0).
Implementation
static DateTime firstDayOfMonth(DateTime date) => _date(date.isUtc, date.year, date.month);