The determining qualification for retirement and the benefit provided under Section 21546 and calculating benefits payable upon death before retirement other than that provided under Section 21546, a year of service shall be credited for each year during which the member was employed throughout the year on a part-time basis and was engaged in his or her duties the full amount of time he or she was required by his or her employment to be so engaged. In calculating service to determine the qualification, credit for fractional years shall be granted to the extent of the fraction derived by dividing the time during which the member was engaged in his or her duties within the year, by the time he or she was required by his or her employment to be so engaged.