If you don't want to buy a clock for your desk...
And you don't want run in windowed mode so you can use Windows' clock...

How about launching FFXIV via Steam? Its free, works with fullscreen mode and you can push SHIFT+TAB to see the time.

With everything that's changing in FFXIV, I wouldn't really want the developers wasting time to add something that's so easy to find another solution to and is, for most people, just going to take up screen real-state.