12 or 24 hour, I have a hard time reading the RX3 clock.
Aerostich's Clip Clock works great on the RX3 bar. Its easy to read and it is 12 hour. And you don't need to consult the owner's manual to reset it.
Aerostich points out that it is not fully waterproof but I have used mine for years on multiple bikes, and it keeps on ticking. Does seem to need a new battery every year or so. I also put a new O ring between the movement and the mount when I change the battery.
http://www.aerostich.com/clock-clip.html