Different rules apply to counting service for purposes of eligibility, benefit accrual, and vesting.
Refer to your summary plan description to see which method is used by your plan. Generally speaking, if your employer provides a plan that covers your position, you must be permitted to become a participant if you have reached age 21 and have completed 1 year of service. Even if you work part-time or seasonally, you cannot be excluded from the plan on the grounds of age or service if you meet this service standard. You must be permitted to begin to participate in the plan no later than the start of the next plan year or 6 months after meeting the requirements of membership, whichever is earlier. You should be aware, however, that your employer may provide one or more plans covering different groups of employees or may exclude certain categories of employees from coverage under any plan.