/** 
	 * string $value holding timestamp in internal timestamp format. Does not have to be valid for strToTime.
	 */
	function validateTimestamp($value) {
		$minValue = ($this->minValue==null)? "0000-00-00 00:00:00": $this->minValue;
		$maxValue = ($this->maxValue==null)? "9999-12-31 23:59:59": $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);
	}