title: Join/Merge two pandas dataframes and filling
body: <p>I have two pandas dataframes both holding irregular timeseries data. </p>
  
  <p>I want merge/join the two frames by time.</p>
  
  <p>I also want to forward fill the other columns of frame2 for any "new" rows that were added through the joining process. How can I do this?</p>
  
  <p>I have tried:</p>
  
  <pre><code>df = pd.merge(df1, df2, on="DateTime")
  </code></pre>
  
  <p>but this just leave a frame with matching timestamp rows.</p>
  
  <p>I would be grateful for any ideas!</p>