Time keeps changins

I have a Panasonic DVD Recorder DMR-ES40V which works great when it’s not automatically changing the time on itself. For no reason that I can see, the machine just changes the time and in turn, then screws up any preset recordings I have entered. One minute it will be set at the correct time, then the next time I look at it, the time jumps ahead four/five hours or more/less. :confused: Any ideas why it might do this?