title: FormatException: Invalid date format 2018-08-1
body: <p>If I write, in DartPad, <code>print(DateTime.parse("2012-12-12"));</code> it works. So why doesn't it work when I do the following (it reports an "invalid date format"):</p>
  
  <pre class="lang-dart prettyprint-override"><code>  for (int i = 1; i &lt;= 28; i = i + 7) {
      _activities[DateTime.parse('2012-12-$i')] = activityNames.toList();
    }
  </code></pre>