Employer's obligation to fund a pension plan for the time period when employees were qualified to participate but the plan was not yet established. For example, a pension plan is established at XYZ Co. in 1985. Because John Smith started at the firm in 1975, he would have a past service credit for his 10 years of service before the plan started. For funding purposes, annual contributions are broken down into future service benefits and past service benefit. Past service liability is not funded entirely in the initial year of a plan, primarily because it would be too expensive, and the IRS requires that deductions be spread over 10 years.