I'm not going to be mean and disqualify anyone that accidentally falls foul of some arbitrary rule, so don't worry too much about timezones - but of course get your game submitted before the end date.
The reason for keeping submissions private (which doesn't mean you can't share with testers) is so that very polished games don't overwhelm and demotivate amateur authors. In addition, revealing lots of hype about a game means that games developed earlier can campaign for more votes.