Did we shortchange this employee on his seniority when we rehired him?

Q: “An employee left the company in good standing to pursue other interests in August of 2012. We rehired him six months later and started him as a new employee. He lost his previous seniority of 13+ years. My position is that his original seniority date should have been adjusted to reflect his time away from work, and that he should not have returned with new employee status. What do other companies do, and is there any legal support for my position?” – T., New Jersey