(iv) For purposes of determining an employee's nonforfeitable percentage of accrued benefits derived from employer contributions, a plan, after calculating an employee's period of service in the manner prescribed in this paragraph, may disregard any remaining less than whole year, 12-month or 365-day period of service. Thus, for example, if a plan provides for the statutory three to seven year graded vesting, an employee with a period (or periods) of service which yields 3 whole year periods of service and an additional 321-day period of service is twenty percent vested in his or her employer-derived accrued benefits (based solely on the 3 whole year periods of service).