My second job, I agreed on a start date having forgotten that I had a week-long cruise booked for the very next week. It had been arranged a year or so before by a friend and just was not on my mind. So I started week 3 of that job with a -40 hour PTO balance.

Companies have a lot of flexibility in deciding how the accruals work. But, as you said, the important part is that when it accrues, at that point it's a legally-obligated right for you to either receive that time or be paid for it.