Logically, the best I can come up with is date of original hire - therefore a guy coming in from an airline that has been absorbed who was hired on say June 6 1989 would fit in between existing guys hired on say June 1 1989 and June 10 1989.
Personally, I favour a merit based system - where the best guys get promoted first, rather than simply serving time.
The problem is, how does one make that method fair and impartial?