title: Negative timestamp -- does it makes sense in PHP?
body: <p>According to the <a href="http://www.php.net/manual/en/function.time.php" rel="noreferrer">manual</a>, PHP uses the Unix Epoch (00:00:00 UTC on 1 January 1970) to generate its timestamps.</p>
  
  <p>Given this definition, I believe that it is reasonable to work with negative timestamps, when the target date is before the Unix Epoch. But, since time is a complicated matter in programming, I want to know if there is any caveat in using negative timestamps in PHP.</p>