class RulesDateOffsetProcessor

class RulesDateOffsetProcessor extends RulesDataProcessor {
    
    /**
     * Overrides RulesDataProcessor::form().
     */
    protected static function form($settings, $var_info) {
        $settings += array(
            'value' => '',
        );
        $form = array(
            '#type' => 'fieldset',
            '#title' => t('Add offset'),
            '#collapsible' => TRUE,
            '#collapsed' => empty($settings['value']),
            '#description' => t('Add an offset to the selected date.'),
        );
        $form['value'] = array(
            '#type' => 'rules_duration',
            '#title' => t('Offset'),
            '#description' => t('Note that you can also specify negative numbers.'),
            '#default_value' => $settings['value'],
            '#weight' => 5,
        );
        return $form;
    }
    
    /**
     * Overrides RulesDataProcessor::process().
     */
    public function process($value, $info, RulesState $state, RulesPlugin $element) {
        $value = isset($this->processor) ? $this->processor
            ->process($value, $info, $state, $element) : $value;
        return RulesDateOffsetProcessor::applyOffset($value, $this->setting['value']);
    }
    
    /**
     * Intelligently applies the given date offset in seconds.
     *
     * Intelligently apply duration values > 1 day, i.e. convert the duration
     * to its biggest possible unit (months, days) and apply it to the date with
     * the given unit. That's necessary as the number of days in a month
     * differs, as well as the number of hours for a day (on DST changes).
     */
    public static function applyOffset($timestamp, $offset) {
        if (abs($offset) >= 86400) {
            // Get the days out of the seconds.
            $days = intval($offset / 86400);
            $sec = $offset % 86400;
            // Get the months out of the number of days.
            $months = intval($days / 30);
            $days = $days % 30;
            // Apply the offset using the DateTime::modify and convert it back to a
            // timestamp.
            $date = date_create("@{$timestamp}");
            $date->modify("{$months} months {$days} days {$sec} seconds");
            return $date->format('U');
        }
        else {
            return $timestamp + $offset;
        }
    }

}