Help on generating absence quota

Hi all SAP Experts,

How can i make a requirement such that for 1st year of service, this employee who joined on 1st June 2010 gets 14 * (5/12) annual leave but

while for first day of 2nd year of service, he gets 15 annual leave,

then for first day of 3rd year of service, he gets 16 annual leave?

In this case, i seem to be not able to use seniority because it will calculate from his joining date and i wanted at his first day of 2nd year of service, he gets constant 15 annual leave.

Only those new employees who just joined in first year, need to pro rata like 14 * (5/12)

Anyone can advise? Thanks

Regards,

HongLing