FERS creditable service can include:
(for more specific details, see the related articles listed below)

• generally the period of time from each date of appointment to date of separation (when deductions were paid into the retirement system)
  

• Leave without pay (LWOP) if it does not exceed six months within a calendar year
  

• Part-time service
  

• Intermittent service (time actually served, WAE [when-actually-employed]) - up to 260 days a year.
  

• workers compensation is creditable if the employee returns to duty
  

• breaks in service of up to 3 days
  

• Deposit service  (time employee worked for federal government but there were no dedeductions for FERS retirement, if performed before January 1, 1989). 
  

• Military service
  

• Unused sick leave (only used to extend length of service for FERS employees who transferred from CSRS and have a CSRS component to their retirement annuity).