ot/cron Schedule previousAsTime() incorrect result

[jamend]

Schedule's previousAsTime() method seems to return incorrect results at times, ex. the
result is after the given time.

$pattern = '30 17 * * 2 *';
$schedule = Ot_Cron_Schedule::fromCronString($pattern);
$input = 1370966140; // Tue, 11 Jun 2013 11:55:40 -0400
$result = $schedule->previousAsTime($input); // Tue, 11 Jun 2013 17:30:00 -0400
assert($result < $input, 'result datetime should be before the input datetime');