This excerpt taken from the PBI 10-K filed Mar 1, 2007.
Employers Accounting for Postretirement Benefits Other Than Pensions, actual results that differ from our assumptions and estimates are accumulated and amortized over the estimated future working life of the plan participants and will therefore affect pension expense recognized and obligations recorded in future periods. Net pension expense is based primarily on current service costs, interest costs and the returns on plan assets. In accordance with this approach differences between the actual and expected return on plan assets are recognized over a five-year period and as a result future pension expense will be impacted when these previously deferred gains or losses are recorded. We use a measurement date of December 31 for all of our retirement plans. See new accounting pronouncement below for the effect of SFAS No. 158,