system/classes/crontab.php

<?php
  namespace Habari;

  class CronTab {

    static function get_cronjob( $name )
    {
      if ( is_int( $name ) ) {
        $cron = DB::get_row( 'SELECT * FROM {crontab} WHERE cron_id = ?', array( $name ), '\Habari\CronJob' );
      }
      else {
        $cron = DB::get_row( 'SELECT * FROM {crontab} WHERE name = ?', array( $name ), '\Habari\CronJob' );
      }
      return $cron;
    }

    static function delete_cronjob( $name )
    {
      $cron = self::get_cronjob( $name );
      if ( $cron ) {
        return $cron->delete();
      }
      return false;
    }

    static function add_cron( $paramarray )
    {
      // Delete any existing job with this same name
      if($job = CronTab::get_cronjob($paramarray['name'])) {
        $job->delete();
      }

      $cron = new CronJob( $paramarray );
      $result = $cron->insert();

      //If the new cron should run earlier than the others, rest next_cron to its strat time.
      $next_cron = DB::get_value( 'SELECT next_run FROM {crontab} ORDER BY next_run ASC LIMIT 1', array() );
      if ( intval( Options::get( 'next_cron' ) ) > intval( $next_cron ) ) {
        Options::set( 'next_cron', $next_cron );
      }
      return $result ? $cron : false;
    }

    static function add_single_cron( $name, $callback, $run_time, $description = '' )
    {
      $paramarray = array(
        'name' => $name,
        'callback' => $callback,
        'start_time' => $run_time,
        'end_time' => $run_time, // only run once
        'description' => $description
      );
      return self::add_cron( $paramarray );
    }

    static function add_hourly_cron( $name, $callback, $description = '' )
    {
      $paramarray = array(
        'name' => $name,
        'callback' => $callback,
        'increment' => 3600, // one hour
        'description' => $description
      );
      return self::add_cron( $paramarray );
    }

    static function add_daily_cron( $name, $callback, $description = '' )
    {
      $paramarray = array(
        'name' => $name,
        'callback' => $callback,
        'increment' => 86400, // one day
        'description' => $description
      );
      return self::add_cron( $paramarray );
    }

    static function add_weekly_cron( $name, $callback, $description = '' )
    {
      $paramarray = array(
        'name' => $name,
        'callback' => $callback,
        'increment' => 604800, // one week (7 days)
        'description' => $description
      );
      return self::add_cron( $paramarray );
    }

    static function add_monthly_cron( $name, $callback, $description = '' )
    {
      $paramarray = array(
        'name' => $name,
        'callback' => $callback,
        'increment' => 2592000, // one month (30 days)
        'description' => $description
      );
      return self::add_cron( $paramarray );
    }

  }

?>