FERS Annuity
The FERS annuity is based on a combination of the length of an employee's Federal service eligible for FERS retirement (referred to as "creditable Federal service", which may or may not equal the actual length of Federal service) and the average of the employee's highest three salaries earned during service (commonly shortened to "high-3").
The annuity does not begin until one full calendar month has passed since the employee's retirement. Thus, an employee retiring on the last day of a month (June 30, for example) will have his/her annuity begin on August 1 (as the entire month of July will have passed), but an employee retiring during a month (July 1, for example) will not have his/her annuity begin until September 1 (as July will not be a full month passed, but August will be).
Eligibility for Social Security benefits and TSP withdrawals are covered by the regulations for those plans.
Read more about this topic: Federal Employees Retirement System