startDate<T extends Every>  static method 
Returns the start date considering the given every base process.
If every is a LimitedEvery, the limit will be passed on.
Implementation
static DateTime startDate<T extends Every>(
  T every,
  DateTime date, {
  required DateTime? limit,
}) {
  if (every is! LimitedEvery) return every.startDate(date);
  return every.startDate(date, limit: limit);
}