title: dart DateTime.add add 1 hours from a specific date
body: <p>I Would have a dart questions.</p>
  
  <pre><code>print(DateTime(2020,03,12).add(new Duration(days: 17)));
  print(DateTime(2020,03,12).add(new Duration(days: 18)));
  </code></pre>
  
  <p>Results:</p>
  
  <p>2020-03-29 00:00:00.000</p>
  
  <p>2020-03-30 01:00:00.000</p>
  
  <p>I don't understand the second result. Why 01:00:00?</p>
  
  <p>This is the result running on Flutter test and dartpad.dev online, although if I run it from flutter application it shows 00:00:00 correctly. Why?</p>
  
  <pre><code>print(DateTime(2019,03,12).add(new Duration(days: 17)));
  
  print(DateTime(2019,03,12).add(new Duration(days: 18)));
  </code></pre>
  
  <p>shows:</p>
  
  <p>2019-03-29 00:00:00.000</p>
  
  <p>2019-03-30 00:00:00.000</p>
  
  <pre><code>print(DateTime(2021,03,12).add(new Duration(days: 17)));
  
  print(DateTime(2021,03,12).add(new Duration(days: 18)));
  </code></pre>
  
  <p>shows:</p>
  
  <p>2021-03-29 01:00:00.000</p>
  
  <p>2021-03-30 01:00:00.000</p>