the clock shouldnt have any problems with working when no signal
Ive had my phone in flight mode many times and theres no noticable delay in how the clock functions
see it as a household clock as long as theres power to it, it will keep on ticking.
atleast on the european tp2s and cant see why it would be different with any of the other models
the only problem should be if you travel between time zones if theres no signal theres nothing to tell the phone to update the time for the given time zone.
but as long as you stay in the same time zone there shouldnt be any problems
