title: Adding/subtracting a second to a UTC timestamp
body: <p>I have some timestamp data in the form of "2013-07-31T13:31:29" that I need to add a second to. I was having some issue with "clock add" so my plan was to convert the time into epoch time and then increment it. When trying to do this I noticed that the times don't seem to match each-other.</p>
  
  <pre><code>Timestamp: 2013-07-31T13:31:29
  Epoch time: 1375252289
  GMT: Wed, 31 Jul 2013 06:31:29 GMT
  </code></pre>
  
  <p>This timestamp was generated via the TCL code below:</p>
  
  <pre><code># timeMinusMilli == 2013-07-31T13:31:29
  set epoch [ clock scan $timeMinusMilli -gmt 0 ]
  </code></pre>
  
  <p>Now, maybe I'm just confused, but I would think that 2013-07-31T13:31:29 would be Wed, 31 Jul 2013 1:31:29, not 6:31:29.</p>