U.S. Office of Personnel Management - Ensuring the Federal Government has an effective civilian workforce

The employing office (or its agent if it has made arrangements for some other office to handle its TCC accounts) bills you for each pay period (generally each month) you are covered. The initial bill may include more than one month's coverage if more than one month has passed since the effective date. Payments are due after the month during which you are covered in accordance with the schedule and procedures established by the employing office.