Unfortunately, these difficult economic times increase the likelihood that employees will be moved to part-time employment.  From an employee benefits perspective, moving someone to part-time can have an impact on things like eligibility and vesting so employers and plan administrators have to be aware of the various requirements in the regulations and how they define "part-time."

First, there is the 1000 hours per year requirement in IRS Code Sections 410(a)(3)(A) and 411(a)(5)(A).  For retirement plans, this means that eligibility and vesting for a "year of service" are satisfied when someone works 1000 hours in the plan year.  So going to "part time" for retirement plan purposes would really be governed by the 1000 hours measurement and working more than 1000 hours in a year would still constitute a full year of service even if it was not a 40-hour work week.

On the other hand, not all benefit plans are under that same requirement.  Major medical plans are governed in part by the eligibility requirements of 105(h), and 105(h)(3)(B) permits exclusion for part-time employees without specifically defining "part-time."  The 1000 hours does not apply to these plans, but IRS commentary provides that part-time means someone who has customary employment of less than 25 hours per week.  So eligibility to continue to participate in those plans may be protected by working an average of 25 hours per week, which is more than the 1000 hours for retirement plans.

Add to that the confusion over Section 125 plan and Section 129 Dependent Care Assistance plans.  Like retirement plans, they are subject to discrimination testing, which would seem to require the 1000 hour measurement (Section 129 actually references 410(a) and the 10000 hours).  However, Section 79 life insurance plans follow the same definition as Section 105(h), which means they permit exclusion for part-time employees without considering the 1000 hours of service.  Add to this the new requirement for Section 403(b) plans (a type of retirement plan, unusually for non-profit institutions) that retains the 1000 hours but provides that the plan generally does not have to be offered to employees expected to work fewer than 20 hours per week and you can see where there would be some considerable confusion over what it means to be "part-time."

But I think that this confusion ends up being the answer.  From a benefits perspective, the definition of part time depends on the benefit plan at issue.  Taking an employee from full to part- time has an impact on benefit eligibility, vesting and years of service credit.  There is a uniform lack of uniformity which means that moving a participant to part time requires you to look at each plan to see what happens.  An employee may lose health benefits but still be entitled to a vesting year of service under the 401(k) plan.  They may stay in the cafeteria plan for discrimination purposes, but lose life insurance.  Good administrators will take the time to understand and explain to employees the impact of the reduction in hours, and of course, check with their legal counsel to make sure they are doing it correctly.