/** 
	 * string $value holding date in internal date format. Does not have to be valid for strToTime.
	 */
	function validateDate($value) 
	{
		$minValue = ($this->minValue==null)? "0000-00-00": $this->minValue;
		$maxValue = ($this->maxValue==null)? "9999-12-31": $this->maxValue;

		if ($value === null && $this->minLength > 0)
			return $this->errorTooShort.$this->minLength;
			
		if ($value === null) return null;

		return 	$this->validate_min_max($value, $minValue, $maxValue);
	}