firstDayInPreviousMonth method

DateTime firstDayInPreviousMonth([
  1. int firstDay = 1
])

上月第一天

firstDay - 每月起算日期，assert(firstDay > 0 && firstDay < 29)

Implementation

DateTime firstDayInPreviousMonth([int firstDay = 1]) {
  return lastDayInPreviousMonth(firstDay).firstDayInMonth(firstDay);
}