protected function copyAndNormalize(/* ... $args */) {
			if ($this->utcTimezone === null)
				$this->utcTimezone = new DateTimeZone('UTC');

			$args = func_get_args();

			foreach ($args as $i => $dt)
				if ($dt instanceof DateTime) {
					$dt = clone $dt;

					$dt->setTimezone($this->utcTimezone);

					$args[$i] = $dt;
				} else
					throw new InvalidArgumentException('All arguments must be DateTime objects');

			return $args;
		}