Employee benefits

while accounting for employee benefits as per AS-15, in the defined benefit plan, we determine the present value of defined benefit obligation and the related service cost and past service costs. The rate used  for discounting is the rate of market yeild from govt. bonds. my question is From which base year should the discount rate(pv of re1 @ X%) be applied that is to say whether the rate applied say x% be from the date of inception of the plan or on year to year basis?