I can't think of a single word that expresses this idea and where the meaning of the sign would be clear.
I think the best you're going to do is something like Charles W's suggestion: a two-word title like "Ahead/Behind" or "Early/Late". I'd add that you should probably give some notation to indicate which is a plus and which is a minus. Like, I'd probably make the heading "Ahead(+)/Behind(-)" (or the other way around, whichever way you want to express it).
If it's for technical people, it's common to refer to deviations as a "delta", so you could call it "Schedule Delta". But you'd still have the issue of clarifying whether a positive number means you're ahead or behind.
Unless you have an established convention, I'd make positive mean ahead of schedule, just because we normally think of positive numbers as being good and negative numbers as being bad. Not always, but usually.