How to Subtract Minutes

I want to send a reminder email.I don't want to use cron on Linux/Unix/BSD box or Scheduled Tasks on Windows.

I'm trying to subtract 15 minutes from the current time.

here is my code so far (doesn't work):

$days   = date("j",time());
$months = date("n",time());
$years  = date("Y",time());
$hours  = date("G",time());
$mins   = (date("i",time()));
$secs   = date("s",time());
$mins   = $mins-15;