My 2008 Suzuki DR-650 motorcycle didn't come with a built-in clock, and because I already have a lot of adventure touring instruments mounted to the handlebar, I decided to mount an old digital watch to the left side mirror stalk. Since I already had an old digital watch, my cost was less than $5 with about 1/2 hour of effort.
The construction is quite simple - just epoxy a tee-nut to the rear of a digital to create a threaded mount, and then attach the watch to the mirror stalk with a small screw and a rubber insulated metal clamp.
This idea should work with most other motorcycles lacking a built-in clock, including most vintage bikes. The watch LCD display is easily read in the daylight, and I don't usually ride at night.
Step 1: The Stuff You'll Need
Rubber insulated metal clamp - Measure your motorcycle's mirror stalk diameter where you plan to attach the watch so you can determine what size rubber insulated metal clamp to buy. If its near 3/8 or 1/2 inch diameter, or other standard clamp size (they come in 1/8 inch increments) you're in luck - just buy the clamp matching your motorcycle's mirror stalk diameter. Unfortunately, my mirror stalk diameter was 0.44 inches so I had to buy the next size up clamp (1/2 inch, Gardner Bender PPR-1550) which means I'll also need to enlarge the stalk diameter with a nylon sleeve.
Nylon sleeve - If needed, buy a nylon sleeve to enlarge the stalk diameter to match the clamp size. I my case a sleeve 0.38 ID and 1/2 OD, 1 inch long did the trick. I found it at Home Depot.
Digital Watch - I used an old Timex. Of course the watch should be waterproof.
Screw and tee-nut - 1/2 long screw and tee nut sized to match the rubber insulated metal clamp's mounting hole. In my case, for the 1/2 clamp, I used a 1/4-20 screw and tee-nut. Smaller thread size will be needed for a smaller clamp and vise-versa.
Loctite thread locker, 2 part epoxy, E6000 or similar household adhesive.
Metal file, screwdriver, hacksaw
Step 2: Assembly
File down the tee-nut barbs (for safety's sake) and epoxy it the back of digital watch. Avoid getting epoxy on the back plate screws. Let the epoxy cure overnight. Optionally cut off the watch band mounting lugs.
If you need to enlarge the mirror stalk diameter, remove a 90 degree section of the nylon sleeve using a hacksaw and secure to mirror stalk with adhesive as shown in the photos. I used E6000 household adhesive because it's a bit less runny than epoxy.
Apply Loctite thread locking compound to the 1/2 long screw and attach the watch to mirror stalk using the rubber insulated metal clamp as shown in the photo. Position the watch for good viewing when sitting on the motorcycle and tighten in place.
That's it, you're done!