This comment was originally written by @seaneagan

Would probably want to change Duration.toString() to match ISO-8601 or whatever format is chosen for Duration.parse.

Added Area-Library, Library-Core, Triaged labels.

Removed Type-Defect label.
Added Type-Enhancement label.

Issue #19891 has been merged into this issue.

Yep, this would have been nice to have today.

I also needed this today.

Worked on this and added support for it in https://dart-review.googlesource.com/c/sdk/+/118566.

I would like to have a toIso8601String method in addition to parsing.

@lohnn why was this never merged? Or, maybe better: what can I do to get it merged?

I guess my PR just didn't get any attention. I can take a shot at updating it and then report back here. Then we can see what we need to do to get it merged?

@lohnn Would definitely be useful!
Two remarks on the code:

1. for the time being I've copied over the code into my project. I've modified the _parseTime() method like this:

static int _parseTime(String duration, String timeUnit) {
  final timeMatch = RegExp('(\\d+)$timeUnit').firstMatch(duration);
  if (timeMatch == null) {
    return 0;
  }
  return int.parse(timeMatch.group(1));
}

2. Since you're working in the actual Duration class already, maybe making the static method a factory constructor might be an option? I actually don't know if this is considered more "elegant", just wanted to point out that that's something a mere extension currently can't do.

I am definitely in favor of this, though I'm not sure what format it should parse. For my use case, Duration should be able to parse back the output of it's .toString() method.

var d = const Duration(days: 1, hours: 1, minutes: 33, microseconds: 500);
print(d.toString()); // 25:33:00.000500

I can see the argument for using the ISO 8601 standard though, perhaps both should be implemented.