add method
Adds elapsed calendar microseconds without applying timezone rules.
Implementation
LocalDateTime add(Duration duration) {
final days = _floorDiv(duration.inMicroseconds, _microsecondsPerDay);
final ticks =
time.microseconds + duration.inMicroseconds % _microsecondsPerDay;
return LocalDateTime(
date.addDays(days + ticks ~/ _microsecondsPerDay),
LocalTime.fromMicroseconds(ticks % _microsecondsPerDay),
);
}