Florida PTO - Is there a law that dictates how/when it is used by an employee?

My wife is an RN for a company in Florida. They are making her use her accrued PTO if she leaves early for the day (even though she has someone else cover her time). This seems unfair. There has got to be some kind of law that states that she can use her PTO at her discretion, not the companies. Please respond only if you can back up your answer with facts rather than opinions. Thanks.