title: Subset a dataframe between two time periods
body: <p>If I have an example dataframe:</p>
  
  <pre><code>Date &lt;- c("05/12/2012 05:17:00", "05/12/2012 06:10:00", "05/12/2012 06:12:00", "05/12/2012 06:14:00", 
        "06/12/2012 05:25:00", "06/12/2012 06:55:00", "06/12/2012 06:19:00", "06/12/2012 08:00:00",
        "07/12/2012 05:00:00", "07/12/2012 05:19:00", "07/12/2012 06:04:00",
        "07/12/2012 06:59:00")
  Date &lt;- strptime(Date, "%d/%m/%Y %H:%M")
  a &lt;- sample(12)
  hour &lt;- as.numeric(format(Date, "%H"))
  min &lt;- as.numeric(format(Date, "%M")) / 60
  hours_mins &lt;- hour + min
  
  df1 &lt;- data.frame(Date,a,hour, min, hours_mins, stringsAsFactors = FALSE)
  </code></pre>
  
  <p>I wish to be able to subset my dataframe leaving only data (on any day) between the hours of 05:15 and 06:15.</p>
  
  <p>I converted the hours and mins to a decimal variable, and hoped I might be able to do something like:</p>
  
  <pre><code>df1[df1$hours_mins &gt;= '5.25' &amp; df1$hours_mins &lt; '6.25']
  </code></pre>
  
  <p>... but alas this doesn't work.  Does anyone have any suggestions?</p>