startMonthDay function
Gets the first day of the first week of a month. Commonly this is not the first of the month, unless that happens to fall on the start day of the week.
Optional startingDayOfWeek
defaults to sunday (1). See startingDayOfWeekToInt
for more information
Implementation
DateTime startMonthDay(DateTime date, {int startingDayOfWeek = 1}) {
final DateTime d = date._firstDayOfWeek(startingDayOfWeek);
return DateTime(d.year, d.month);
}