53 Pa. Stat. § 23571
The time of service herein specified, namely twenty years, shall be computed from the time of the first or original employment; said employment to consist of service to such city of the second class, 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 who shall be employed after the passage of this act and who shall have been employed by such city prior to the passage of this act. If an employe shall have enlisted or shall have been drafted to serve in the Army or Navy of the United States in time of war, or shall have been drafted to serve in the Army or Navy of the United States in time of peace, such service in the Army or Navy of the United States shall be credited in full to the service record of such employe as service to such city of the second class.
53 P.S. § 23571