I just discovered that I can enter durations as reals, not just integers. The little arrow buttons pretty much always, as far as I can remember, work with integers, so I just figured that was what I had to do.
I have to admit I'm not sure what, if anything, should be done about this. I'm inclined to say the arrows shouldn't be there if you're allowing more free numeric input, but that's probably silly. I just can't think of another instance where arrows and decimal points have gone together. Failing memory, perhaps.