53 Pa. Stat. § 13444
The time of service herein specified, namely, twenty years, or in case of the lesser pension, fifteen years or more but less than twenty years, shall be computed from the time of the first or original employment; said employment to consist of service either to such cities or to the county, or other public service paid out of the city treasury, or both, as aforesaid, and need not be continuous: Provided, That in no case shall a period of more than eighteen years be credited to the service record of any person or persons who shall be employed after the passage of this act, and who shall have been employed by the said cities prior to the passage of this act. No pensions shall be paid under the provisions of this act, however until, after January first, one thousand nine hundred and seventeen.
The time of service shall include service with a school district of the first class coterminous with the city or county of the first class if the person claiming such credit shall, within one year after the effective date of this amending act, or within one year after joining the pension fund in the case of any person who may hereafter join the pension fund, pay into the fund the contributions he or she would have paid had he or she been employed during the period of credited service by the city or county at the same salary received from the school district. In no case shall credited service with a school district exceed twelve years.
For the purpose of determining the total number of years or months of service or contribution under this act, any period of fifteen days or more but less than a month beyond the total number of full months shall be considered a "month of service".
53 P.S. § 13444