title: unknown timezone name in R strptime/as.POSIXct
body: <p>Where can I find a list of all legal time names for R function <code>as.POSIXct</code>?</p>
  
  <p><code>as.POSIXct("1970-01-01",tz="CST")</code> generates a warning that "CST" (Central Standard Time) is unknown.</p>