I was wondering the same thing. I have a set which has that heading. I've googled Promotion Service Date and can't find it mentioned specifically. I don't think it's the same thing as Eligible for Promotion. However since promotion is part of the phrase maybe it shouldn't be used/keyed.
Some in the military are given an early promotion or "frocked." Some in the military have qualified for their next promotion and are waiting on a list for a spot. And sometimes in the military when you're promoted, you have a mininum additional service requirement.
Good question. I'm waiting for an answer, too. I'll go by Anna's last answer for now which dealt with Eligible for Promotion only. Promotion does sound like a future event, therefore not keyed.