Why does clock on 2004 ninja 636 keep resetting?

Check your battery or terminals. The clock generally resets when the computer losses power.

You may just have a loose terminal!

Also, when the battery gets very weak, sometimes this will cause your clock to reset.

Any time the battery dies, your clock will most likely reset as well (or stay at the time it last died/ lost connection).

If this doesn't work, you can take off the upper fairings and have a look at the wiring harness. Check for corrosion and any loose connections. If you find corrosion, you can pop the pins out using a bobby pin or a jewler's set screw driver.

Clean the pins off, something acidic helps, such as water with some lemon juice in it. Wipe them off the fill the little holes with dielectric grease before putting the pins back in. This will help prevent corrosion coming back in the future.

You may also find a corroded wire. Cut the corroded part of the wire, and add a new section - do it the proper way: solder any parts of the added wire together to seal the connection and use heat shrink wrap on these new sections, and at the terminal, to make a better sealed connection and prevent any dirt/ moisture getting in.

Hopefully this is not linked to a more complex electrical problem.

And, lastly, there could be something wrong with your computer, ecu, etc. However, there is usually a simple solution to these problems.