SimpleDB timestamp for insertion

I am working on a game on the iPhone that will use SimpleDB to store game data, and I would like to be able to time out some entries in the database.

I believe that I will have some kind of cron job that will look at the timestamp of each record, and if it is larger than X, I will select this record (change var in it so that it is again available for user selection) .

My problem: I do not know how to unify timestamps. What can I do if the user has a system clock and is disconnected for a couple of hours or even days? I would like to get the system time from SimpleDB servers when I insert, but I believe that I read that SimpleDB does not provide any timestamp.

My only thought is that all clients get a timestamp from some random source on the Internet at startup, and then determine timestamps based on that time and system ticks, but I would prefer to keep this internal for my program and AWS how can I not guarantee that I will provide a third party time source and when.

Can someone suggest a solution to this problem, or perhaps warn me about the available timestamp supported by AWS servers? I'm still at the design stage, so I haven't worked much with AWS messages, can their response packets contain time somewhere?

Thank! -Skyler