Just call HRSSC , Shared Services Center, at 1-877-477-3723. They will let you know what your union anniversary date is as well as when the forms must be received to make the 10 day window. It's referred to as a 10/20/10 window. For example, if your anniversary date is 5/11/XX ; you must cancel between 4/21/XX and 5/1/XX. If you look, you'll see that the LAST date listed in the example is 10 days BEFORE your actual anniversary date. So, at the end of it all, it's within 10 days BEFORE the 10 days that precede you anniversary date. The 10 and 10 make the 20. They will walk you through the process.