Employment contracts often contain language that requires a legal claim to be filed within a specific period of time -- a period that is shorter than the statute of limitations set by the state legislature or by Congress. Very often a court will enforce a contract-based shorter limitation period. In Boaz v. FedEx (6th Cir 08/06/2013) the court said it would not enforce the shorter period when it comes to claims under the federal Fair Labor Standards Act (FLSA) or Equal Pay Act (EPA).
The reasoning was pretty simple: FLSA and EPA claims cannot be waived by employees, and a shortened limitations period is a waiver.
Of course, this case does not necessarily apply to any other statutory claims an employee might have (Title VII, for example).
As Jon Hyman at Ohio Employer's Law Blog and others point out, employers will continue drafting employment agreements that shorten the statute of limitations. Usually that will work.