title: Merging dataframes of different length by matching dates
body: <p>I have two dataframes of different lengths (n=3012 and 3008) that I need to merge based upon date.  I have tried using the merge and rbindfill functions but had no success.  The common thread between the two dataframes is date_time however both df2 does not have all of the same values as df1.</p>
  
  <p>df1</p>
  
  <pre><code>ID date_time Q_cfs Data_Code
  68 2016-11-01 00:00:00   353         P
  69 2016-11-01 00:15:00   356         P
  70 2016-11-01 00:30:00   357         P
  71 2016-11-01 00:45:00   356         P
  72 2016-11-01 01:00:00   358         P
  73 2016-11-01 01:15:00   355         P
  </code></pre>
  
  <p>df2</p>
  
  <pre><code>ID  stage           date_time
  1  4.82 2016-11-01 00:00:00
  2  4.83 2016-11-01 00:15:00
  3  4.84 2016-11-01 00:30:00
  4  4.85 2016-11-01 00:45:00
  5  4.86 2016-11-01 01:00:00
  6  4.87 2016-11-01 01:15:00
  </code></pre>
  
  <p>I tried using merge (below) but it did not work since the column lengths are different.</p>
  
  <pre><code>DF_New&lt;- merge(df1,df2, by.x = df1$date_time, by.y = df2$date_time)
  </code></pre>
  
  <p>I also tried using rbind.fill (below) but it replaced all the values of df2 with .</p>
  
  <pre><code>DF_New &lt;- rbind.fill(df1,df2)
  </code></pre>
  
  <p>Any suggestions?</p>