Calculate when a cron job will be executed then next time

1 * * * * (every hour at xx:01)
2 5 * * * (every day at 05:02)
0 4 3 * * (every third day of the month at 04:00)
* 2 * * 5 (every minute between 02:00 and 02:59 on fridays)

//Totaly made up language
next = getTimeNow();
next.addMinutes(1) //so that next is never now
done = false;
while (!done) {
  if (cron.minute != '*' && next.minute != cron.minute) {
    if (next.minute > cron.minute) {
      next.addHours(1);
    }
    next.minute = cron.minute;
  }
  if (cron.hour != '*' && next.hour != cron.hour) {
    if (next.hour > cron.hour) {
      next.hour = cron.hour;
      next.addDays(1);
      next.minute = 0;
      continue;
    }
    next.hour = cron.hour;
    next.minute = 0;
    continue;
  }
  if (cron.weekday != '*' && next.weekday != cron.weekday) {
    deltaDays = cron.weekday - next.weekday //assume weekday is 0=sun, 1 ... 6=sat
    if (deltaDays < 0) { deltaDays+=7; }
    next.addDays(deltaDays);
    next.hour = 0;
    next.minute = 0;
    continue;
  }
  if (cron.day != '*' && next.day != cron.day) {
    if (next.day > cron.day || !next.month.hasDay(cron.day)) {
      next.addMonths(1);
      next.day = 1; //assume days 1..31
      next.hour = 0;
      next.minute = 0;
      continue;
    }
    next.day = cron.day
    next.hour = 0;
    next.minute = 0;
    continue;
  }
  if (cron.month != '*' && next.month != cron.month) {
    if (next.month > cron.month) {
      next.addMonths(12-next.month+cron.month)
      next.day = 1; //assume days 1..31
      next.hour = 0;
      next.minute = 0;
      continue;
    }
    next.month = cron.month;
    next.day = 1;
    next.hour = 0;
    next.minute = 0;
    continue;
  }
  done = true;
}

//Shorter more loopy version
next = getTimeNow().addMinutes(1);
while (true) {
  if (cron.month != '*' && next.month != cron.month) {
    next.addMonths(1);
    next.day = 1;
    next.hour = 0;
    next.minute = 0;
    continue;
  }
  if (cron.day != '*' && next.day != cron.day) {
    next.addDays(1);
    next.hour = 0;
    next.minute = 0;
    continue;
  }
  if (cron.weekday != '*' && next.weekday != cron.weekday) {
    next.addDays(1);
    next.hour = 0;
    next.minute = 0;
    continue;
  }
  if (cron.hour != '*' && next.hour != cron.hour) {
    next.addHours(1);
    next.minute = 0;
    continue;
  }
  if (cron.minute != '*' && next.minute != cron.minute) {
    next.addMinutes(1);
    continue;
  }
  break;
}

<?php

// Works with predefined scheduling definitions
$cron = CronCronExpression::factory('@daily');
$cron->isDue();
$cron->getNextRunDate();
$cron->getPreviousRunDate();

// Works with complex expressions
$cron = CronCronExpression::factory('15 2,6-12 */15 1 2-5');
$cron->getNextRunDate();

class myMiniDate {
	var $myTimestamp;
	static private $dateComponent = array(
									'second' => 's',
									'minute' => 'i',
									'hour' => 'G',
									'day' => 'j',
									'month' => 'n',
									'year' => 'Y',
									'dow' => 'w',
									'timestamp' => 'U'
								  );
	static private $weekday = array(
								1 => 'monday',
								2 => 'tuesday',
								3 => 'wednesday',
								4 => 'thursday',
								5 => 'friday',
								6 => 'saturday',
								0 => 'sunday'
							  );

	function __construct($ts = NULL) { $this->myTimestamp = is_null($ts)?time():$ts; }

	function __set($var, $value) {
		list($c['second'], $c['minute'], $c['hour'], $c['day'], $c['month'], $c['year'], $c['dow']) = explode(' ', date('s i G j n Y w', $this->myTimestamp));
		switch ($var) {
			case 'dow':
				$this->myTimestamp = strtotime(self::$weekday[$value], $this->myTimestamp);
				break;

			case 'timestamp':
				$this->myTimestamp = $value;
				break;

			default:
				$c[$var] = $value;
				$this->myTimestamp = mktime($c['hour'], $c['minute'], $c['second'], $c['month'], $c['day'], $c['year']);
		}
	}


	function __get($var) {
		return date(self::$dateComponent[$var], $this->myTimestamp);
	}

	function modify($how) { return $this->myTimestamp = strtotime($how, $this->myTimestamp); }
}


$cron = new myMiniDate(time() + 60);
$cron->second = 0;
$done = 0;

echo date('Y-m-d H:i:s') . '<hr>' . date('Y-m-d H:i:s', $cron->timestamp) . '<hr>';

$Job = array(
			'Minute' => 5,
			'Hour' => 3,
			'Day' => 13,
			'Month' => null,
			'DOW' => 5,
	   );

while ($done < 100) {
	if (!is_null($Job['Minute']) && ($cron->minute != $Job['Minute'])) {
		if ($cron->minute > $Job['Minute']) {
			$cron->modify('+1 hour');
		}
		$cron->minute = $Job['Minute'];
	}
	if (!is_null($Job['Hour']) && ($cron->hour != $Job['Hour'])) {
		if ($cron->hour > $Job['Hour']) {
			$cron->modify('+1 day');
		}
		$cron->hour = $Job['Hour'];
		$cron->minute = 0;
	}
	if (!is_null($Job['DOW']) && ($cron->dow != $Job['DOW'])) {
		$cron->dow = $Job['DOW'];
		$cron->hour = 0;
		$cron->minute = 0;
	}
	if (!is_null($Job['Day']) && ($cron->day != $Job['Day'])) {
		if ($cron->day > $Job['Day']) {
			$cron->modify('+1 month');
		}
		$cron->day = $Job['Day'];
		$cron->hour = 0;
		$cron->minute = 0;
	}
	if (!is_null($Job['Month']) && ($cron->month != $Job['Month'])) {
		if ($cron->month > $Job['Month']) {
			$cron->modify('+1 year');
		}
		$cron->month = $Job['Month'];
		$cron->day = 1;
		$cron->hour = 0;
		$cron->minute = 0;
	}

	$done = (is_null($Job['Minute']) || $Job['Minute'] == $cron->minute) &&
			(is_null($Job['Hour']) || $Job['Hour'] == $cron->hour) &&
			(is_null($Job['Day']) || $Job['Day'] == $cron->day) &&
			(is_null($Job['Month']) || $Job['Month'] == $cron->month) &&
			(is_null($Job['DOW']) || $Job['DOW'] == $cron->dow)?100:($done+1);
}

echo date('Y-m-d H:i:s', $cron->timestamp) . '<hr>';

function parse_crontab($time, $crontab)
         {$time=explode(' ', date('i G j n w', strtotime($time)));
          $crontab=explode(' ', $crontab);
          foreach ($crontab as $k=>&$v)
                  {$v=explode(',', $v);
                   foreach ($v as &$v1)
                           {$v1=preg_replace(array('/^*$/', '/^d+$/', '/^(d+)-(d+)$/', '/^*/(d+)$/'),
                                             array('true', $time[$k].'===', '(1<='.$time[$k].' and '.$time[$k].'<=2)', $time[$k].'%1===0'),
                                             $v1
                                            );
                           }
                   $v='('.implode(' or ', $v).')';
                  }
          $crontab=implode(' and ', $crontab);
          return eval('return '.$crontab.';');
         }
var_export(parse_crontab('2011-05-04 02:08:03', '*/2,3-5,9 2 3-5 */2 *'));
var_export(parse_crontab('2011-05-04 02:08:03', '*/8 */2 */4 */5 *'));