Provoke Gregorian at the outset with year missing from their calendar? (4)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

A charade of G for the initial letter of ‘Gregorian’ and O AD. The year zero does not exist in the AD calendar; its first year was AD1. Which a while ago led to long and fruitless discussions about whether the new millennium started on January 1st 2000 or January 1st 2001. There are more important things to worry about.