title: Round a POSIX date and time (posixct) to a date relative to a timezone
body: <p>I want to round a POSIXct down to the day, relative to a specific timezone.</p>
  
  <p>If I try</p>
  
  <pre><code>round(as.POSIXct("2013-03-05 23:00:00 EST"), "day")
  </code></pre>
  
  <p>It returns</p>
  
  <pre><code>2013-03-06
  </code></pre>
  
  <p>Which makes sense, in that when it's 23:00:00 EST on 2013-03-05 in EST5EDT, it's already 2013-03-06 in UTC. Logically, what I want to do is:</p>
  
  <pre><code>round(as.POSIXct("2013-03-05 23:00:00 EST"), "day", tz="EST5EDT")
  </code></pre>
  
  <p>That is, "round this date and time to the nearest day, relative to the EST5EDT time zone". Unfortunately, <em>round</em> doesn't take a time zone parameter.</p>