My suggestion would be to add some kind of a way for the players to also be able to book their schedules in comparison to that of the GM. For example, the GM may set the game time to be from a range of days, say, 1st till 7th of a month. The players then "Check" the time frames when they can attend. This is all relayed to the GM and the players with the time converted to their respective time zones automatically. It would be a major help to be able to adjust this in-game when dealing with multiple people from different time zones in the world. This, or a way to post pre-converted time and dates into the chat inside the forums of the campaign.