“Rate of earnings”: The actual rate upon which the earnings of an employee are calculated at any time, as certified in a written notice, on file with the board, by the governing body of the municipality, or instrumentality, or participating instrumentality. For periods during which the employee did not participate but is entitled to creditable service, the monthly earnings shall be considered to be the earnings in the position for each calendar year divided by the number of months of creditable service in that year.