When you get iPhone is disabled or try again in a bazillion minutes try putting a sim card in and let the cellular data sync up the date and time for you + turn phone off and on after it syncs.
If that does not work then you need to do a software update via iTunes in DFU mode.
And if that still doesn't work then you need to restore the phone as a last resort
Alternatively the client can put the disabled phone plugged into their computer if they have synced it before and reset that disabled message so they can enter in the passcode.
When battery is unplugged date and time is reset.
Date and time is synced automatically if set date and time is set to be updated automatically in settings and is connected to internet via wifi or sim card cellular data.
Plugging the phone into iTunes syncs date and time too but only if the iPhone is unlocked or been synced with that computer before.
Note that the phone has to not be in airplane mode and/or cellular data enabled for some of these options to work.