A recent lawsuit in Illinois has lessons that apply to any employer covered by the Family Medical Leave Act (FMLA). After terminating an employee for using too much time off, the employer wound up in court unable to show the employee or the court how they determined the 12 month period that determines eligibility for the 12 weeks of medical leave.
The court then determined the 12 months in the most favorable way possible for the employee.  The result? The case against the employer goes forward.
It might be a very good time to review your FMLA policy. Does it specify how the 12 month period is calculated?
If not, or even if it does, here is an article that discusses the case and the various ways the 12 month period can be determined. The author strongly favors one method over the others.