title: Converting date times in POSIXct gives screwy result?
body: <p>I'm doing some extracting of data from a database, and running the results through rehsape2.  For some reason this is mangling the POSIXct datetime stamps into numeric.  No Problem I think, you can just turn them back, except I'm an hour out.</p>
  
  <p>Here's a minimal example</p>
  
  <pre><code>foo&lt;-as.POSIXct("2011-04-04 14:18:58")
  as.numeric(foo)     #gives 130192318
  bar&lt;-as.POSIXct(as.numeric(foo), 
                  tz=Sys.timezone(),
                  origin=as.POSIXct(
                    strptime("1970-01-01 00:00:00", "%Y-%m-%d %H:%M:%S", tz="UTC")))
  as.numeric(bar)     #gives 130192318 identical !
  foo     #Gives "2011-04-04 14:18:58 BST"
  bar     #Gives "2011-04-04 13:18:58 UTC"
  </code></pre>
  
  <p>Obviously foo and bar are numerically identical, but R thinks foo needs to be displayed as BST and bar as UTC.  How do I get both displayed as BST.  This doesn't work either;</p>
  
  <pre><code>as.POSIXct(bar, tz="BST")   #still gives "2011-04-04 13:18:58 UTC"
  </code></pre>