26 CFR §1.410(a)-9T

1. (a)-(b) [Reserved]
2. (c)Eligibility to participate.
   1. (1)[Reserved]
   2. (2)Determination of one-year period of service.
      1. (i)[Reserved]
      2. (ii)For purposes of section 410(a)(1)(B)(i), a “2-year period of service” shall be deemed to be “2 years of service.”
3. (d)Vesting
   1. (1)General rule.
      1. (i)-(iii) [Reserved]
      2. (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).