I guess it is broken. I'm no expert but I can't see the DAB radio part affecting the clock timing part
It sets itself to the right time ? In which case it must be picking up the timing radio signal. And afterwards it loses time, in which case it is either no longer using the radio signal after setting, or it fails to receive it.
Sounds to me as if it is using some kind of inadequate back up system to keep time when it can't find the radio signal.
Since it corrects when you put the radio on, maybe it only looks for/finds the timing radio signal when the radio is switched on ?
Take it back to the shop under warranty, and complain.