Welcome to the Apple Community.
Without being obvious it means just what it says, however the date and time should be set automatically for you by the network time server (but the average user wouldn't be aware of this)
One reason the date and time might not be set is when access to the server is blocked. The network time server is accessed through TCP port 123 and you may need to check your router isn't blocking this port.
However if nothing has been changed on your network it might be nothing more than a glitch, so you might be best to try restarting the Apple TV by unplugging it for a few moments before trying anything else.