next method
Implementation
DateTime next(int day, bool isForWeek) {
if (isForWeek) {
if (day == weekday) {
return add(const Duration(days: 7));
} else {
return add(
Duration(
days: (day - weekday) % DateTime.daysPerWeek,
),
);
}
} else {
return add(Duration(days: day));
}
}