Unify Date.fromString and support more Iso 8601.

frog/leg/lib/date_helper.dart

Index: frog/leg/lib/date_helper.dart
diff --git a/frog/leg/lib/date_helper.dart b/frog/leg/lib/date_helper.dart
index 9c19ac66ac2a579df578a3155b076de40a243f11..630d8fc3414ee08086b7a8db30c066d0768b6f38 100644
--- a/frog/leg/lib/date_helper.dart
+++ b/frog/leg/lib/date_helper.dart
@@ -2,6 +2,15 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+String fourDigits(int n) {
+  int absN = n.abs();
+  String sign = n < 0 ? "-" : "";
+  if (absN >= 1000) return "$n";
+  if (absN >= 100) return "${sign}0$absN";
+  if (absN >= 10) return "${sign}00$absN";
+  if (absN >= 1) return "${sign}000$absN";
+}
+
 String threeDigits(int n) {
   if (n >= 100) return "${n}";
   if (n > 10) return "0${n}";