Get the Calendar
of your ZonedDateTime
and then call the GetDaysInMonth(year, month)
method:
ZonedDateTime mstNow = now.InZone(mst);
CalendarSystem calendar = mstNow.Calendar;
LocalDate mstLastDayOfCurrentMonth = new LocalDate(
mstNow.Year, mstNow.Month, calendar.GetDaysInMonth(mstNow.Year, mstNow.Month));