title: Change timezone in a POSIXct object
body: <p>Playing with dateTimes and timezone can be tricky in R.
  Here is my question: I want to change the time-zone on a <code>POSIXct</code> object</p>
  
  <pre><code>R) data &lt;- data.frame(x=c(1,2),dateTime=as.POSIXct(c("2010-03-11 03:30:00.432","2010-03-15 03:30:00.432"),format="%Y-%m-%d %H:%M:%S",tz="America/Montreal"))
  R) data
    x            dateTime
  1 1 2010-03-11 03:30:00
  2 2 2010-03-15 03:30:00
  R) str(data)
  'data.frame':   2 obs. of  2 variables:
   $ x       : num  1 2
   $ dateTime: POSIXct, format: "2010-03-11 03:30:00" "2010-03-15 03:30:00"
  </code></pre>
  
  <p>But if I want to change the timezone, the only thing I found is:</p>
  
  <pre><code>R) data$dateTime2 = format(data$dateTime,tz="Europe/Paris")
  R) str(data)
  'data.frame':   2 obs. of  3 variables:
   $ x        : num  1 2
   $ dateTime : POSIXct, format: "2010-03-11 03:30:00" "2010-03-15 03:30:00"
   $ dateTime2: chr  "2010-03-11 09:30:00" "2010-03-15 08:30:00"
  </code></pre>
  
  <p>Or</p>
  
  <pre><code>R) data$dateTime2 = as.POSIXlt(data$dateTime,tz="Europe/Paris")
  R) str(data)
  'data.frame':   2 obs. of  3 variables:
   $ x        : num  1 2
   $ dateTime : POSIXct, format: "2010-03-11 03:30:00" "2010-03-15 03:30:00"
   $ dateTime2: POSIXlt, format: "2010-03-11 09:30:00" "2010-03-15 08:30:00"
  </code></pre>
  
  <p>God (or somebody on SO) knows why it doesn't work with <code>POSIXct</code></p>
  
  <pre><code>R) data$dateTime2 = as.POSIXct(data$dateTime,tz="Europe/Paris")
  R) str(data)
  'data.frame':   2 obs. of  3 variables:
   $ x        : num  1 2
   $ dateTime : POSIXct, format: "2010-03-11 03:30:00" "2010-03-15 03:30:00"
   $ dateTime2: POSIXct, format: "2010-03-11 03:30:00" "2010-03-15 03:30:00"
  </code></pre>
  
  <p>Do I need to convert to <code>character</code> and cast back to <code>POSIXct</code> ?</p>