title: fastPOSIXct equivalent for converting non-UTC to UTC
body: <p>Hi I have a character vector (<code>rr</code>) that is several million in length, and it represents time and date stamps in the format <code>%Y-%m-%d %H:%M:%S</code> recorded in Australia/Sydney.</p>
  
  <p>How do get a POSIXct object (quickly) that represents this.</p>
  
  <p>I have found <code>fastPOSIXct</code> in the <code>fasttime</code> package, but for this to be accurate, it requires the original character string to be in GMT/UTC, (which mine is not) and then converted back into the correct timezone using the <code>tz</code> arguement...</p>
  
  <pre><code>&gt; head(rr)
  [1] "2009-05-01 10:01:00" "2009-05-01 10:02:00" "2009-05-01 10:03:00" "2009-05-01 10:04:00"
  [5] "2009-05-01 10:05:00" "2009-05-01 10:06:00"
  
  &gt; as.POSIXct(head(rr),tz="Australia/Sydney")
  [1] "2009-05-01 10:01:00 EST" "2009-05-01 10:02:00 EST" "2009-05-01 10:03:00 EST"
  [4] "2009-05-01 10:04:00 EST" "2009-05-01 10:05:00 EST" "2009-05-01 10:06:00 EST"
  </code></pre>
  
  <p>The above line takes ages if doing it on the full set of data...so any speed improvements would be appreciated. Thanks.</p>