title: How to add/subtract time from a POSIXlt time while keeping its class in R?
body: <p>I am manipulating some <code>POSIXlt</code> DateTime objects.  For example I would like to add an hour:</p>
  
  <pre><code>my.lt = as.POSIXlt("2010-01-09 22:00:00")
  new.lt = my.lt + 3600
  new.lt
  # [1] "2010-01-09 23:00:00 EST"
  class(new.lt)
  # [1] "POSIXct" "POSIXt" 
  </code></pre>
  
  <p>The thing is I want <code>new.lt</code> to be a <code>POSIXlt</code> object.  I know I could use <code>as.POSIXlt</code> to convert it back to <code>POSIXlt</code>, but is there a more elegant and efficient way to achieve this?</p>