bool isBefore(DateTime other)

Returns true if this occurs before other.

The comparison is independent of whether the time is in UTC or in the local time zone.

DateTime berlinWallFell = new DateTime(1989, 11, 9);
DateTime moonLanding    = DateTime.parse("1969-07-20 20:18:00");

assert(berlinWallFell.isBefore(moonLanding) == false);

Source

bool isBefore(DateTime other) {
  return _value < other._value;
}