title: Adding days to Date in javascript / typescript gives completely wrong date
body: <p>I have this very simple code, and it works sometimes but other times it gives the complete wrong date: </p>
  
  <pre><code>  addDays(date: Date, days: number): Date {
      console.log('adding ' + days + ' days');
      console.log(date);
      date.setDate(date.getDate() + days);
      console.log(date);
      return date;
    }
  </code></pre>
  
  <p>Here is an example of the output: </p>
  
  <p>Works!</p>
  
  <pre><code>adding 7 days
  error-chart.component.ts:118 Tue Aug 02 2016 00:00:00 GMT+0200 (Vest-Europa (sommertid))
  error-chart.component.ts:120 Tue Aug 09 2016 00:00:00 GMT+0200 (Vest-Europa (sommertid))
  </code></pre>
  
  <p>Stops working:</p>
  
  <pre><code>adding 14 days
  error-chart.component.ts:118 Tue Aug 02 2016 00:00:00 GMT+0200 (Vest-Europa (sommertid))
  error-chart.component.ts:120 Thu Mar 02 2017 00:00:00 GMT+0100 (Vest-Europa (normaltid))
  </code></pre>
  
  <p>somehow jumps 2 years </p>
  
  <pre><code>adding 14 days
  error-chart.component.ts:118 Thu Jul 07 2016 00:00:00 GMT+0200 (Vest-Europa (sommertid))
  error-chart.component.ts:120 Thu Jun 14 2018 00:00:00 GMT+0200 (Vest-Europa (sommertid))
  </code></pre>
  
  <p>now 4 years!</p>
  
  <pre><code>adding 14 days
  error-chart.component.ts:118 Thu Jun 14 2018 00:00:00 GMT+0200 (Vest-Europa (sommertid))
  error-chart.component.ts:120 Thu Apr 14 2022 00:00:00 GMT+0200 (Vest-Europa (sommertid))
  </code></pre>