Calculate when a cron job will be executed then next time

Question

I have a cron \"time definition\"

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          


        

Answer 1

For anyone interested, here's my final PHP implementation, which pretty much equals dlamblin pseudo code:

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>';

Answer 2

This is basically doing the reverse of checking if the current time fits the conditions. so something like:

//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;
}

I might have written that a bit backwards. Also it can be a lot shorter if in every main if instead of doing the greater than check you merely increment the current time grade by one and set the lesser time grades to 0 then continue; however then you'll be looping a lot more. Like so:

//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;
}