Wouldn't a better definition of peacful be the number of days/years where a war wasn't going on somewhere?
By this criterion, the only century that has a chance of having a few years where a war/genocide/etc was NOT waged somewhere in the world is the XX century.
The previous centuries - not a chance.
The concept of peace as the normal state and war as the exception is a modern invention. In the past, war was the normal state and peace, merely a transitory interval between wars.
Also - this criterion is FAR less accurate than the number/intensity of atrocities, for it puts the equal sign between wars/genocides of vastly different scale, vastly different cost in human suffering and death.
And only by using such immense generalizations can it equate the XX century to the previous ones, when it comes to atrocities.