Do they give you a place for free-form comments? If so, I'd put it in there. Then either leave the specific question blank or fill in a best guess. If they don't give a place for comments, my inclination would be to leave it blank and explain if and when you get as far as an interview. If the computer won't accept blank, I'd put in a best guess, and again, explain at the interview.
The worst case I can see is if they're trying to fill people in to specific slots, and giving the wrong times would rule you out. You don't say what the job is. Most jobs either expect people to work regular business hours, or if they have shifts, they typically move people around anyway. Everybody I know who does "shift work" is getting "shifted" all the time.