title: Reliable way to detect if a column in a data.frame is.POSIXct
body: <p>R has <code>is.vector</code>, <code>is.list</code>, <code>is.integer</code>, <code>is.double</code>, <code>is.numeric</code>, <code>is.factor</code>, <code>is.character</code>, etc. Why is there no <code>is.POSIXct</code>, <code>is.POSIXlt</code> or <code>is.Date</code>?</p>
  
  <p>I need a reliable way to detect <code>POSIXct</code> object, and <code>class(x)[1] == "POSIXct"</code> seems really... dirty.</p>