Dartで時間を文字列にするにはintlを利用する ref: http://qiita.com/sh4869/items/cc64511cec78fb208c2a

exmaple.dart

import 'package:intl/intl.dart';
import 'package:intl/date_symbol_data_local.dart';
import 'dart:core';

void main(){
  var format = new DateFormat.Hms();
  var dateString = format.format(new DateTime.now());
  print(dataString);
}