How much can the incubation time vary? (Primarily duck eggs)

Hi y'all,
I'm am really, really hoping to hatch some duck eggs this spring () and I'm trying to hatch them right after my semester ends for school, which is a little later than I'd probably choose to hatch otherwise, but I want to be there as much as possible when my babies are little, so I'm waiting till school's out. Anyway...I'm trying to have them hatch Monday, May 28. So that would mean setting on April 30th, right? But I want to make sure I'm there for the hatch: do eggs ever/commonly hatch early? If so, how much earlier?

Thanks for your help! If I do get the eggs I'm sure I'll be here a lot for advice! I've had a broody hen once before, but she took care of most of it for me.