In computing the length of service of a contributor for retirement purposes, full credit shall be given to each original member for each year of service rendered to the city prior to January first of the year the retirement system was established. This shall include the service of a city official whose compensation was in the form of fees collected by his office, and shall also include the services of employes paid directly by such city official out of such fees. As soon as practicable, the retirement board shall issue to each original member a certificate, certifying the aggregate length of his service prior to January first of the year retirement system was established. Such certificate shall be final and conclusive as to his prior service, unless thereafter modified by the board upon application of the member. The time during which a member is absent from service without pay shall not be counted in computing the service of a contributor unless allowed by the city council and approved by the board.