Clear credit in theory should post a day or two after the charge posts. However, things happen and Amex screws up. Unlike other offers that say wait 6-8 weeks or 90 days for the credit to appear, the Clear offer says to wait 4 weeks before contacting Amex. So just to be safe, purchase Clear at least 4 weeks before you plan to cancel the card, incase a manual credit is required on Amex's end.