You will need a motor like the one pictured and yes I sell it (without a case...the one in the picture $10 that includes shipping) and I really hate saying that because just about every post...It sounds like I'm trying to sell someone something. That's really not the case ( I just about break even on this motor) you can also get it from China ($8.70) but I buy about 500 of these motors a year and let members have them for $1.00 over my cost and that cover my ink/tape/paper/screw that fits the threaded end . Now that we got that out of the way on to the important part.
This motor will reverse itself once it can't go any farther in one direction. So once it hits the side of your clock (I'm assuming you have sides) it will go in the other direction and keep repeating that movement.
I will try to get you a video to show exactly what I'm taking about