Doc Brown (played by Christopher Lloyd) wears a Seiko A826 Training Timer watch in the movie Back To The Future.

The watch has a unique rotating bezel that activates the different modes. It has an adjustable velcro strap with Seiko-signed buckle. The most unique feature is the the remote finger trigger, used for setting split times, which can clearly be seen in the screenshot.

Doc Brown is wearing his Seiko on the right wrist. On the left wrist he has a silver calculator watch. See a picture of that watch and more Back to the Future watches on Pocket Calculator Show.

The Seiko A826 Training Timer is sometimes offered on eBay. Currently there is this one from top-rated seller George Papay.

Thanks to Dell Deaton and George Papay