The only problem is that we know one thing discord came after equestria was born and we don't know when that time came so we canot make a exact date.
but if he did not rule that long when did luna and celestia apaer because we know they where not full grown mares i think they would be around 20 years then if that is the states but what is the background story of luna and celestia we don't know mutch about that do we
so that leaves me out with the question when did they apear