title: How to make time difference in same units when subtracting POSIXct
body: <p>I want to subtract to POSIXct. I can do this but depending on the first row (i guess?)  the difference will be in seconds or minutes.  Below you can see the first diff is in seconds and the second diff is in minutes because I changed the time difference in the first row:</p>
  
  <pre><code>#diff in seconds because 1st row time diff is small?
  t1&lt;- as.POSIXct(c("2015-02-02 20:18:03 00:00:00", "2015-02-02 20:17:02 00:00:00"),"GMT")
  t2&lt;- as.POSIXct(c("2015-02-02 20:18:02 00:00:00","2015-02-02 20:18:02 00:00:00"),"GMT")
  d&lt;-data.frame(t1= t1, t2= t2)
  d$t1-d$t2
  
  
  #diff in seconds because 1st row time diff is larger?
  t1&lt;- as.POSIXct(c("2015-02-02 20:13:03 00:00:00", "2015-02-02 20:17:02 00:00:00"),"GMT")
  t2&lt;- as.POSIXct(c("2015-02-02 20:18:02 00:00:00","2015-02-02 20:18:02 00:00:00"),"GMT")
  d&lt;-data.frame(t1= t1, t2= t2)
  d$t1-d$t2
  </code></pre>
  
  <p>results:</p>
  
  <pre><code>&gt; #diff in seconds because 1st row time diff is small?
  &gt; t1&lt;- as.POSIXct(c("2015-02-02 20:18:03 00:00:00", "2015-02-02 20:17:02 00:00:00"),"GMT")
  &gt; t2&lt;- as.POSIXct(c("2015-02-02 20:18:02 00:00:00","2015-02-02 20:18:02 00:00:00"),"GMT")
  &gt; d&lt;-data.frame(t1= t1, t2= t2)
  &gt; d$t1-d$t2
  Time differences in secs
  [1]   1 -60
  &gt; 
  &gt; 
  &gt; #diff in seconds because 1st row time diff is larger?
  &gt; t1&lt;- as.POSIXct(c("2015-02-02 20:13:03 00:00:00", "2015-02-02 20:17:02 00:00:00"),"GMT")
  &gt; t2&lt;- as.POSIXct(c("2015-02-02 20:18:02 00:00:00","2015-02-02 20:18:02 00:00:00"),"GMT")
  &gt; d&lt;-data.frame(t1= t1, t2= t2)
  &gt; d$t1-d$t2
  Time differences in mins
  [1] -4.983333 -1.000000
  </code></pre>
  
  <p>I would like the difference to ALWAYS be in seconds no matter what the first row difference is. Is there a way to make this happen?</p>
  
  <p>Thank you.</p>