datetime.dart

import 'package:intl/intl.dart';

void main() {
  var data = "12/26/2021 3:16 PM";

  String dateTime = getFormattedDateFromFormattedString(
      value: data,
      currentFormat: "MM/dd/yyyy hh:mm a",
      desiredFormat: "yyyy-MM-ddTHH:mm:ss.mmmuuuZ");
  
  print(dateTime); //2021-12-15T15:16:00.000Z
}

getFormattedDateFromFormattedString(
    {required value,
    required String currentFormat,
    required String desiredFormat,
    isUtc = true}) {
  DateTime? dateTime = DateTime.now();
  if (value != null || value.isNotEmpty) {
    try {
      dateTime = DateFormat(currentFormat).parse(value, isUtc);
    } catch (e) {
      print("$e");
    }
  }
  return dateTime!.toIso8601String();
}