I have only noticed those date errors on PHP 5 myself, so its hard to say what will happen when you switch it back to PHP 5 mode. It depends on if your host changed anything for that date setting. PowerGallery requires PHP 5 since it uses some features (such as SimpleXML) that aren't available in PHP 4.
If you still get the error in PHP 5, try looking in to the date_default_timezone_set() function function.date-default-timezone-set.php. Looks like that is another way to set the timezone on a per page basis. Not as clean as a site wide fix, but would probably do the trick in your case.