title: VB.NET & Linq: Sum datatable column and Group by date in another column
body: <p>I have a typedDataset (ds) and its datatable(dt). The datatable has the columns <code>Date</code>(ttmmyyy) and <code>Worktime</code>. I want to sum the Worktime by month. I already accomplished to sum the Worktime with</p>
  
  <pre><code>Dim sum = dt.AsEnumerable().Sum((Function(x) x.Worktime))
  </code></pre>
  
  <p>I need help with group this by month.</p>