iPhone: How to fix a cheat based on changing the device's date? Is there a server I can get the time from?

You can use an NTP server that tells you the time.

Right until someone uses a proxy server to figure out what NTP server you are trying to connect to, and then spoofs the NTP response... as others have mentioned, the response is easy to parse, and thus easy to forge.

A better solution would be to use something like Google App Engine (free), to send your app some kind of encrypted response that would boil down to the server time. If the client time deviated from that by too much, the app would simply use the server time for calculations. Then even if they intercept the request they will not easily be able to send back the correct response.

But basically in any networked game, you must always assume the client is lying. The only real way to prevent issues is to have everything go through a server, so a client cannot decide it's suddenly rich and have any other client agree it is so...