Thank you again.
Employers in this circumstance have a surprising amount of discretion. At the present, employers have no legal obligation to provide benefits of any kind. Thus, the law permits employers to determine who is eligible for benefits by any criteria that does not discriminate on the basis of a legally protected trait, such as race, religion, or gender. (i.e., an employer can't offer healthcare only to make employees because the premiums for female employees are too high). Until the Affordable Care Act's employer mandate goes into effect in 2015 (and I'll get to that in a moment), the designation of full time versus part time is purely a construct of the employer.
If your company offers benefits of any kind to employees on the basis of "full time" status, it is entirely up to you how you want to define that term. You may wish to determine full time status by an average of hours worked over a given period of time (e.g. 90 days, 1 year, etc.). If an employee works more than the threshold number of hours which you define, they will be entitled to benefits of whatever kind you choose untiil the next re-evaluation period, during which their hours are examined again.
Ideally, you'd want to include your new definition of full and part time employees n your employee handbook so that your stance is clear, but it is entirely up to where you want to draw the line.
Now, I did mention the Affordable Care Act above and I do want to touch on that again, because that will change the answer here beginning in 2015. Beginning January 2015, employers with 50 or more employees must provide health care to their "full time" employees, defined by the Act as employees who work an average of 30 hours or more per week. The method of calculation for determining whether an employee works 30 or more hours is particularly complicated (here's a useful summary from the Washington Post: http://www.washingtonpost.com/blogs/on-small-business/post/small-business-advice-how-to-count-full-time-and-part-time-employees-under-obamacare/2013/09/27/5466b26c-26e3-11e3-b3e9-d97fb087acd6_blog.html
) but you should be aware that full time and part time will be terms that take on a new legal meaning in about 13 months in terms of an employer's obligations with respect to health care.
Until then, employers need not provide health care or any other benefits, and so when they do, the law affords employers vast discretion to determine who within the company qualifies. Employers are free to make a distinction between full and part time employees and to define the threshold in any manner they choose. Generally speaking, it's best to employ some sort of average over a period of weeks or months, as suggested above, but however you choose to define full and part time status you should incorporate that defintiion into your employee handbook so things are clear.
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