Hi,
well... you have a point, but who's doing that really... cause that affect EVERYTHING in your device, your mails, your calendar, your photos, other apps, this is lame and childish imo just to shunt a game...
if the user is ready to do so, he can just as well shunt the http call you are going to make and return a fake date isn't it? You would have to have a ssl connection established... to prevent this or shuffle/obfuscate/hash the time stamp which is never fun not practical without building custom actions for this.
So yeah, if you have a https server with php do that, get time from a server, but youll face also the possibility that user is offline, and then you are facing more problems and potential issues.
Bye,
Jean