That's how long you have to report pay discrimination. In six short months after a pay discrimination decision was made, you must report that discrimination. Any reports about past pay discrimination or long term discrepancies will be legally ignored.
Oh, you don't feel comfortable asking your co-workers what they make in comparision to you the day you begin your job? Or, wait, you're not even supposed to talk about pay? Too bad...for you.
Thanks to Ledbetter v. Goodyear Tire and Rubber Compay, companies can continue paying women less than their co-workers. As long as the women don't find out about it.