Have you checked the little adjustment screw on the back? If you take it out of the dash, there's a small screw head with some marks next to it so you can set the clock to run faster or slower.
Although they're electrically powered, the clocks are actually clockwork. A little electrical solenoid winds up the coil when it runs low and then the clockwork powers the hands until the coil needs to be wound again. If you've ever wondered what that little 'dr-r-rapp!' noise is in the cabin every 10 mins or so, it's the clock being wound very quickly.
The adjustment screw on the back doesn't adjust the timing directly, but adjusts the frequency of the rewinding, by allowing a greater or lesser time to elapse before the solenoid kicks in and rewinds the coil. If the clockwork had gone altogether, the clock probably wouldn't work at all- or would only work for about 1 minute after each rewind.
My apologies if you already knew this, but I just thought I'd point it out in case. It may save you having to buy a new one if your current one just needs a good fettle!
Best,
Michael