How do I format DateTime with UTC timezone offset?

There is no direct way of getting that kind of date format as of now. There is a work-around.

• Add the intl package
• import it to your file using import 'package:intl/intl.dart';
• Write the following code:

var dateTime = DateTime.now();
var val      = DateFormat("yyyy-MM-dd'T'HH:mm:ss.SSS").format(dateTime);
var offset   = dateTime.timeZoneOffset;
var hours    = offset.inHours > 0 ? offset.inHours : 1; // For fixing divide by 0

if (!offset.isNegative) {
  val = val +
      "+" +
      offset.inHours.toString().padLeft(2, '0') +
      ":" +
      (offset.inMinutes % (hours * 60)).toString().padLeft(2, '0');
} else {
  val = val +
      "-" +
      (-offset.inHours).toString().padLeft(2, '0') +
      ":" +
      (offset.inMinutes % (hours * 60)).toString().padLeft(2, '0');
}
print(val);

What type of date format is this?

This format is UTC + timezone offset.

That +08:00 is the time zone's offset that has already been added.

Seems like DateTime doesn't contain timezone information, so, you can't create a DateTime in a specific timezone. Only the timezone of your system and UTC are available.

Is important to say also that DateTime support timezone offset for parsing, but normalizes it to UTC or local time.

So since this is UTC you can probably format it using toUtc or toLocal and the receiver will be able to parse it.

With that said, you can simply parse it like this:

DateTime.parse("2020-03-26T00:57:08.000+08:00")

What date format is this?
"2020-03-26T00:57:08.000+08:00"

This date-time format follows the RFC 3339 standard, and more generally the ISO 8601 standard. The letter "T" is known as the time designator. The "+08:00" is known as the UTC timezone offset.

I would like to have a date format [...], which has the +08:00 behind

To the date-time, you can append the UTC hour and minute offsets:

// import 'package:intl/intl.dart' as intl show DateFormat;

void main() {
  DateTime now = DateTime.now();
  Duration offset = now.timeZoneOffset;

  // ----------
  String dateTime = now.toIso8601String();
  // - or -
  // String dateTime = intl.DateFormat("yyyy-MM-dd'T'HH:mm:ss").format(now);
  // ----------
  String utcHourOffset = (offset.isNegative ? '-' : '+') +
    offset.inHours.abs().toString().padLeft(2, '0');
  String utcMinuteOffset = (offset.inMinutes - offset.inHours * 60)
    .toString().padLeft(2, '0');

  String dateTimeWithOffset = '$dateTime$utcHourOffset:$utcMinuteOffset';
  print(dateTimeWithOffset);
}

I'm using DateFormat class

DateFormat (https://api.flutter.dev/flutter/intl/DateFormat-class.html) does not format a UTC timezone offset. And while the letter "Z" in the documentation appears to provide the UTC timezone offset, it's reserved, and you can not use DateFormat("Z") as that throws an Unimplemented Error (https://api.flutter.dev/flutter/dart-core/UnimplementedError-class.html). Note that "Z" (pronounced phonetically as "zulu") stands for zero meridian time, and has a UTC timezone offset of +0:00.