Does it start when hired or does it start on day 1 of indoc? I'm assuming most companies have policy that seniority starts when the pilot shows up for training?
I've also heard it starts when training is started. With some guys now being sent to the CPC to get the ATP knocked out, then their seniority may be set up differently at that point.
Oh, sorry, my bad. To the OP: your question depends on what company you're talking about. My answer above was specific to Mesa. (Brain fart on my part, thought this was in the Mesa thread.)