Yeah, and most companies only let you accrue a certain number of hours of time off, typically somewhere between 100-200 hours from what I've seen. Once you hit that limit you simply don't get any more time off. Your boss won't let you take a vacation until next month? Too bad, sucks to be you.

Thankfully the way my current company handles this is about as good as you can get - they cash out balances that are too high a couple times a year at prespecified dates.