You do not have the ability to set the network time. The network does this. You are experiencing one of two problems. You are either not connected to the internet or your router is blocking the date and time server port that apple uses.
To verify network connection: Settings > General > Network
If you are connected to the network with a valid ip try power cycling the router. If this doesnt work you will need to open the ports on your router, which are listed in the following artlicle: