Under the look-back/stability period safe harbor method, an employer would determine each employee’s full-time status by looking back at a defined period of not less than three but not more than 12 consecutive calendar months, as chosen by the employer (the measurement period), to determine whether during the measurement period the employee averaged at least 30 hours of service per week. If the employee were determined to be a full-time employee during the measurement period, then the employee would be treated as a full-time employee during a subsequent “stability period,” regardless of the employee’s number of hours of service during the stability period, so long as he or she remained an employee. For an employee determined to be a full-time employee during the measurement period, the stability period would be a period of at least six consecutive calendar months that follows the measurement period and is no shorter in duration than the measurement period. If the employee were determined not to be a full-time employee during the measurement period, the employer would be permitted to treat the employee as not a full-time employee during a stability period that followed the measurement period, but the stability period could not exceed the measurement period.
Check out the whole thing in all its glory.