model.php

<?php
App::uses('AppModel', 'Model');
/**
 * Voucher Model
 *
 * @property VoucherClaim $VoucherClaim
 * @property Place $Place
 */
class Voucher extends AppModel {

/**
 * Validation rules
 *
 * @var array
 */
	public $validate = array(
		'name' => array(
			'notEmpty' => array(
				'rule' => array('notEmpty'),
				'message' => 'Name is required',
				'allowEmpty' => false,
				'required' => 'create',
			),
		),
		'description' => array(
			'notEmpty' => array(
				'rule' => array('notEmpty'),
				'message' => 'Description is required',
				'allowEmpty' => false,
				'required' => 'create',
			),
		),
		'max_claims' => array(
			'numeric' => array(
				'rule' => array('numeric'),
				'message' => 'Max Claims needs to be a number',
				'allowEmpty' => true,
				'required' => false
			),
		),
		'is_active' => array(
			'boolean' => array(
				'rule' => array('boolean'),
				'message' => 'Is Active is required',
				'required' => 'create',
			),
		),
		'start_time' => array(
			'datetime' => array(
				'rule' => array('datetime'),
				'message' => 'Start Time is required',
				'allowEmpty' => true,
			),
			'valid' => array(
				'rule' => array('dateComparison', array('<', 'end_time')),
				'message' => 'Start time must be before end time'
			)
		),
		'end_time' => array(
			'datetime' => array(
				'rule' => array('datetime'),
				'message' => 'End Time is required',
				'allowEmpty' => true,
			),
			'valid' => array(
				'rule' => array('dateComparison', array('>', 'start_time')),
				'message' => 'End time must be after start time'
			)
		),
	);
}

sample.php

<?php
class SampleBehavior extends ModelBehavior {

/**
 * Validate that one timestamp with a comparison operator is valid
 *
 * Example 1 - Make sure end_time in our model can't be before start_time:
 *		$validate = array('start_time' => array('sanityCheck' => array('rule' => 'dateComparison', 'compareOperator' => '<', 'compareTo' => 'end_time'))));
 *
 * Example 2 - Make sure end_time in our model can't be before start_time::
 *		$validate = array('start_time' => array('sanityCheck' => array('rule' => 'dateComparison', compareOperator' => 'isless', 'compareTo' => 'end_time'))));
 *
 * @param Model $model
 * @param array $params
 */
	public function dateComparison(Model $model, $params, $config = array()) {
		$sourceField = array_pop(array_keys($params));
		$sourceValue = array_pop(array_values($params));

		if (empty($config['compareOperator'])) {
			throw new NodesValidationException('Missing compareOperator key');
		}
		$operator = $config['compareOperator'];

		if (empty($config['compareTo'])) {
			throw new NodesValidationException('Missing compareTo key');
		}
		$targetField = $config['compareTo'];
		if (!isset($model->data[$model->alias][$targetField])) {
			return isset($config['compareAllowEmpty']) ? $config['compareAllowEmpty'] : false;
		}
		$targetValue = $model->data[$model->alias][$targetField];

		try {
			$sourceDate = new DateTime($sourceValue);
		} catch (Exception $e) {
			$model->invalidate($sourceField, $e->getMessage());
			return false;
		}

		try {
			$targetDate = new DateTime($targetValue);
		} catch (Exception $e) {
			$model->invalidate($sourceField, $e->getMessage());
			return false;
		}

		$operator = trim(strtolower($operator));
		switch ($operator) {
			case 'isgreater':
			case '>':
				if ($sourceDate > $targetDate) {
					return true;
				}
				break;
			case 'isless':
			case '<':
				if ($sourceDate < $targetDate) {
					return true;
				}
				break;
			case 'greaterorequal':
			case '>=':
				if ($sourceDate >= $targetDate) {
					return true;
				}
				break;
			case 'lessorequal':
			case '<=':
				if ($sourceDate <= $targetDate) {
					return true;
				}
				break;
			case 'equalto':
			case '==':
				if ($sourceDate == $targetDate) {
					return true;
				}
				break;
			case 'notequal':
			case '!=':
				if ($sourceDate != $targetDate) {
					return true;
				}
				break;
			default:
				throw new NodesValidationException(__d('cake_dev', 'You must define the $operator parameter for Validation::comparison()'));
		}

		return false;
	}

}