title: Drops time when subtracting from POSIXlt object
body: <p>I am trying to subtract an hour (3600 s) from this time object that is defined as 01:00. When I do so, the time component disappears, and I am left with the date only. I need to preserve the time component--how do I do so? This only happens when the result of my subtraction is 00-00.</p>
  
  <pre><code>test &lt;- strptime("2016-09-02_01-00", format =  "%Y-%m-%d_%H-%M", tz = "UTC")
  test
  [1] "2016-09-02 01:00:00 UTC"
  test-3600
  [1] "2016-09-02 UTC"
  </code></pre>