Section 33. For the purposes of this chapter, seniority of a civil service employee shall mean his ranking based on length of service, computed as provided in this section. Length of service shall be computed from the first date of full-time employment as a permanent employee, including the required probationary period, in the department unit, regardless of title, unless such service has been interrupted by an absence from the payroll of more than six months, in which case length of service shall be computed from the date of restoration to the payroll; but upon continuous service following such an absence for a period of twice the length of the absence, length of service shall be computed from the date obtained by adding the period of such absence from the payroll to the date of original employment; provided, however, that the continuity of service of such employee shall be deemed not to have been interrupted if such absence was the result of (1) military service, illness, educational leave, abolition of position or lay-off because of lack of work or money, or (2) injuries received in the performance of duty for which compensation was paid pursuant to chapter one hundred and fifty-two, provided that the employee notifies the appointing authority, in writing, not later than six months after the final payment of compensation aforesaid that he is ready, willing, and able to do his former work and files with said appointing authority a certificate of a registered physician that he is able to perform the duties of his position in an efficient manner, and is restored to the payroll.

If, as a result of a reinstatement made pursuant to section forty-six, a person is restored to employment in a departmental unit other than that in which he formerly held full-time employment as a permanent employee, his length of service shall be computed from the date of his first employment under such reinstatement, but upon continuous service in such unit for three years or twice the length of his absence from the payroll, whichever is greater, his length of service shall be computed as though such earlier employment had been in the departmental unit to which he has been reinstated.

If the employment of such full-time employee is changed through an original or promotional appointment or transfer from one departmental unit of the commonwealth to another under the same appointing authority, or from one departmental unit to another within the same department in a city or town, the length of service of such employee in the unit to which the appointment or transfer is made shall be computed from the date which was used to compute his length of service immediately prior to such appointment or transfer. If the employment of such full-time employee is changed through an original or promotional appointment from one departmental unit of the commonwealth to another not under the same appointing authority, from one departmental unit to another not within the same department in a city or town, from one city or town to another, from a city or town to the commonwealth, or from the commonwealth to a city or town, the length of service of such employee shall be computed from the date of such change of employment, but if the employee completes one year of service in the new employment, from the date which was used to compute the employee's length of service immediately prior to the change of employment.

If the employment of such full-time employee is changed by transfer from one departmental unit of the commonwealth to another not under the same appointing authority, from one departmental unit to another not within the same department in a city or town, from one city or town to another, from a city or town to the commonwealth, or from the commonwealth to a city or town, the length of service of such employee shall be computed in the following manner: (1) if the transfer was made upon the request of the employee, the length of service shall be computed from the date of such transfer, but if the employee completes three years of service in the new employment, from the date which was used to compute the employee's length of service immediately prior to the transfer; (2) if the transfer was not upon the request of the employee, the length of service shall be computed from the date which was used to compute the employee's length of service immediately prior to the transfer. In determining the seniority of a firefighter for the purpose of reduction in rank or reduction in force, his ranking shall be based on his length of service in the fire department in which such reduction is to take place.

The length of service of a permanent employee appointed on less than a full-time basis shall be computed from the date of such appointment, without regard to absences from the payroll which were not voluntary on the part of such employee. Regardless of actual length of service, permanent municipal employees appointed on less than a full-time basis shall, for the purposes of determining seniority, rank below all full-time permanent municipal employees. Permanent state employees appointed on less than a full-time basis shall accrue that portion of the seniority of full-time permanent state employees that their service bears to full-time service.

Any person in such service who has received an appointment from a reemployment list, as set forth in section forty, shall carry forward their seniority as prescribed herein; provided, however, that any lay-offs occurring after a reemployment date shall have such reemployment date as the determining factor in lay-offs by seniority.