In computing the period of service of a member of the Highway Safety Patrol, any member who served on active duty in the Armed Forces of the United States, or who served in maritime service during periods of hostility in World War II, shall be entitled to creditable service at no cost for his service on active duty in the Armed Forces or in such maritime service, provided he entered state service after his discharge from the Armed Forces or entered state service after he completed such maritime service. The maximum period for such creditable service for all military service as defined in this subsection (2) shall not exceed four (4) years unless positive proof can be furnished by such person that he was retained in the Armed Forces during World War II or in maritime service during World War II, by causes beyond his control and without opportunity of discharge. The member shall furnish proof satisfactory to the Board of Trustees of the Public Employees' Retirement System of certification of military service or maritime service records showing dates of entrance into active duty service and the date of discharge. No creditable service shall be granted for any military service or maritime service to a member who qualifies for a retirement allowance in another public retirement system administered by the Board of Trustees of the Public Employees' Retirement System based in whole or in part on such military or maritime service. In no case shall the member receive creditable service if the member received a dishonorable discharge from the Armed Forces of the United States. The credit for military service granted in this subsection shall apply to all persons who have retired from the Highway Patrol and who qualify for credit as outlined above, whether they retired before or after July 1, 2000; but this provision shall not operate to require any back payments of retirement.