title: time zone, period.apply in xts using R
body: <p>i have a bunch of 1 minute data in an <code>xts</code> object with GMT time zone. i call </p>
  
  <pre><code>period.apply (obj, endpoints(obj, "hours"))
  </code></pre>
  
  <p>and for some reason the time zone of my new object changes to EST/EDT, however, when i do <code>str(obj)</code> it still says time zone is GMT.  here is an example:</p>
  
  <pre><code>obj1 &lt;- xts(1:200, order.by=index(ret_1_min_xts)[1:200])
  head(obj1)
                      [,1]
  1986-02-04 14:32:00    1
  1986-02-04 14:33:00    2
  1986-02-04 14:34:00    3
  1986-02-04 14:35:00    4
  1986-02-04 14:36:00    5
  1986-02-04 14:37:00    6
  Warning message:
     timezone of object (GMT) is different than current timezone (). 
  </code></pre>
  
  <p>now do <code>period.apply</code></p>
  
  <pre><code>obj2 &lt;- period.apply(obj1, endpoints(obj1, "hours"), mean)
  head(obj2)
                       [,1]
  1986-02-04 09:59:00  12.5
  1986-02-04 10:59:00  51.0
  1986-02-04 11:59:00 103.5
  1986-02-04 12:59:00 154.0
  1986-02-04 13:26:00 189.5
  
  str(obj1)
  An â€˜xtsâ€™ object from 1986-02-04 14:32:00 to 1986-02-04 18:26:00 containing:
     Data: int [1:200, 1] 1 2 3 4 5 6 7 8 9 10 ...
     Indexed by objects of class: [POSIXct,POSIXt] TZ: GMT
     xts Attributes:  
     List of 2
      $ tclass: chr [1:2] "POSIXct" "POSIXt"
      $ tzone : chr "GMT"
  
  str(obj2)
  An â€˜xtsâ€™ object from 1986-02-04 09:59:00 to 1986-02-04 13:26:00 containing:
     Data: num [1:5, 1] 12.5 51 103.5 154 189.5
     Indexed by objects of class: [POSIXct,POSIXt] TZ: 
     xts Attributes:  
     List of 2
      $ tclass: chr [1:2] "POSIXct" "POSIXt"
      $ tzone : chr "GMT"
  </code></pre>