title: How to fill missing and adjust irregular time intervals in a data.frame in R
body: <p>I have several datasets mostly with 15 min intervals of time. However, some datasets have missing readings (e.g., 3rd row in sample dataset was supposed to be "May 1 2015 00:40AM". In addition, there are some timesteps that are longer than 15 min (e.g., see 3rd and 6th rows)
  How can add the missing time steps so that my Date will continue with 15 min intervals and at the same time adjust those time steps with more than 15 min intervals to 15 min?</p>
  
  <pre><code>s &lt;- data.frame(Date = c(
                         "May 1 2015 00:10AM","May 1 2015 00:25AM",
                         "May 1 2015 00:56AM","May 1 2015 01:10AM",
                         "May 1 2015 01:25AM","May 1 2015 01:41AM",
                         "May 1 2015 01:55AM"),
                          val = c(1:7)
                   )
  </code></pre>
  
  <p>My desired output would be the following:</p>
  
  <pre><code>&gt; s
                  Date val
  1 May 1 2015 00:10AM   1
  2 May 1 2015 00:25AM   2
  3 May 1 2015 00:40AM  NA
  4 May 1 2015 00:55AM   3
  5 May 1 2015 01:10AM   4
  6 May 1 2015 01:25AM   5
  7 May 1 2015 01:40AM   6
  8 May 1 2015 01:55AM   7
  </code></pre>