title: How to select date range with POSIXct type
body: <p>I'm working on a project where I want to track birth totals nine months after another event (wondering if there is a correlation).  It was suggested I use the "lubridate" package to combine MONTH and YEAR in my data frames (which I was seeking to do).</p>
  
  <p>I have successfully used lubridate in both data frames.  The problem is that I only want to select a certain date range from one of those data frames, and I'm having difficulty doing this.  I think it's because the type of data is POSIXct:</p>
  
  <pre><code>str(model.weather)
  'data.frame':   467 obs. of  2 variables:
   $ DATE      : POSIXct, format: "2006-01-01" "2006-01-01" "2006-01-01" "2006-01-01" ...
   $ EVENT_TYPE: Factor w/ 8 levels "Hail","Heavy Snow",..: 2 2 2 2 2 2 3 2 3 3 ...
  </code></pre>
  
  <p>I have tried converting to numeric, but unfortunately that does horrible, horrible things to the data in my DATE variable.  I've tried converting to factor, character, and integer, too - none of them seem to work.</p>
  
  <p>I'm trying to use the "subset" function to select the necessary date range:</p>
  
  <pre><code>model.weather &lt;- subset(model.weather, DATE &gt;= 2006-04-01 | DATE &lt;= 2011-03-01)
  </code></pre>
  
  <p>Unfortunately, this just returns the original data, without doing any filtering.</p>
  
  <p>Can anyone help?</p>