title: Make a data frame of daily values from hourly data
body: <p>So my data looks like this</p>
  <pre><code>station valid           tmpf
  LFMN    1988-01-01 0:00 50.18
  LFMN    1988-01-01 1:00 50
  LFMN    1988-01-01 1:15 50
  LFMN    1988-01-01 1:30 50
  LFMN    1988-01-01 2:00 50
  </code></pre>
  <p>I'd like to have the daily temperature value presented something like</p>
  <pre><code>station day     daily
  LFMN    1988-01-01 51
  LFMN    1988-01-02 49
  </code></pre>
  <p>It seems simple enough but no solutions posted here work. I've tried using zoo, dplyr, plys, and... nothing.
  When I do this</p>
  <pre><code>g %&gt;% mutate(date=as.Date(valid)) %&gt;% group_by(date) %&gt;% summarize(daily=mean(tmpf))
  </code></pre>
  <p>It returns a two column dataset with the correct formats for the day column and everything NA in daily</p>