You want to make sure that all of your creditable service time counts in your retirement calculation. A first step is to check your Official Personnel Folder (OPF) to make sure that it contains all service that is creditable.
The law governing creditable service is 5 U.S. Code 8332, which spells out what service counts and what service doesn’t for inclusion in an annuity calculation. Among them are such obvious ones as active duty military service. But it also includes substitute carrier work for the post office, service with Peace Corps and Vista, volunteer service under the Economic Opportunity Act of 1964, and employment as a United States Capital Guide. Some of these periods of service are creditable only within specific time periods, while others are not. You’ll need to check with your personnel office to see if your work experience can be counted. And if no retirement deductions were taken, you may need to make a deposit for that service to the Civil Service Retirement and Disability Fund.
Also creditable is employment covered by another federal retirement system, such as TVA or the foreign service, as long as you aren’t receiving any retirement benefits for that time under the other system. To receive credit, you’ll have to get a refund of your contributions and deposit it, with interest, in the fund.
It’s advisable to get service records in order well before your retirement. Waiting until the last minute can not only keep you from getting credit for some service but can delay your first annuity payment for a time – sometimes a long, long time – until your work history is sorted out.