A string when not using an output range; void otherwise.
assert(DateTime(Date(2010, 7, 4), TimeOfDay(7, 6, 12)).toISOExtString() == "2010-07-04T07:06:12"); assert(DateTime(Date(1998, 12, 25), TimeOfDay(2, 15, 0)).toISOExtString() == "1998-12-25T02:15:00"); assert(DateTime(Date(0, 1, 5), TimeOfDay(23, 9, 59)).toISOExtString() == "0000-01-05T23:09:59"); assert(DateTime(Date(-4, 1, 5), TimeOfDay(0, 0, 2)).toISOExtString() == "-0004-01-05T00:00:02");
Converts this DateTime to a string with the format YYYY-MM-DDTHH:MM:SS. If writer is set, the resulting string will be written directly to it.