How can I ignore timezone while parsing DateTime?

[adimshev]

final date = DateTime.parse("2021-01-07T18:05:18+0100");
print(date); // 2021-01-07 17:05:18.000Z

I wanna ignore +0100, so result should be 2021-01-07 18:05:18.000Z.
Something like DateTime.parse("2021-01-07T18:05:18+0100", ignoreTimezone: true);.

Related issue #54993

[dart-github-bot]

Summary: The user wants to parse a DateTime string while ignoring the timezone offset, but the current DateTime.parse method doesn't offer an option to do so. They are requesting a new parameter or method to achieve this behavior.

[adimshev]

An alternative way is to copy the regular expression from the DateTime.parse implementation, but this is a bad way.

[lrhn]

I'd do

final _tzRE = RegExp(r"(?:[+\-]\d\d(?::?\d\d)?|(?<![zZ]))$");
DateTime parseDateTimeIgnoreTZ(String input) {
  input = input.replaceFirst(_tzRE, "Z");
  return DateTime.parse(input);
}

This also adds a Z if the input has no time zone, so you always parse into UTC.
If that's not necessary, the RegExp can be made simpler:

final _tzRE = RegExp(r"[+\-]\d\d(?::?\d\d)?$");

Seems to work (always a good idea to test RegExps, you can never tell if they're correct by reading.)

void main() {
  print(parseDateTimeIgnoreTZ("2021-01-07T18:05:18-0100"));
  print(parseDateTimeIgnoreTZ("2021-01-07T18:05:18-01:00"));
  print(parseDateTimeIgnoreTZ("2021-01-07T18:05:18-01"));
  print(parseDateTimeIgnoreTZ("2021-01-07T18:05:18+0100"));
  print(parseDateTimeIgnoreTZ("2021-01-07T18:05:18+01:00"));
  print(parseDateTimeIgnoreTZ("2021-01-07T18:05:18+01"));
  print(parseDateTimeIgnoreTZ("2021-01-07T18:05:18"));
  print(parseDateTimeIgnoreTZ("2021-01-07T18:05:18z"));
  print(parseDateTimeIgnoreTZ("2021-01-07T18:05:18Z"));
}

There is no way to ignore part of the input passed to parse. The point of parse is to parse that input.
If you want to parse something else, you need to give it a different input.

[adimshev]

Thanks for the answer! It's frustrating to have to do this stuff manually.

[lrhn]

It is, but the goal of DateTime.parse is to faithfully represent its input in one of the two ways DateTime can represent points in time: UTC or "local time".
Ignoring part of the input is not being "faithful", so that's not a functionality that fits the class.

Your problem here is that you have a textual representation of a point in time, and you want to represent a different time than that input. The parse function can't really help you with that, so you need to work on its input, not the DateTime created afterwards, which has been put into one of the two buckets that the class actually supports, and has therefore lost some of the information in the original input.

Parsing a date-time string with a time zone into a DateTime is lossful, the result will always be the same point in time in the UTC time zone, no matter which time zone the original was in.