How about making a wonder that does this? Maybe Magna Carta. The steam overlay (shift+tab in game) has a clock, but I see your point. For archery units, how about give them a range of 1 instead of 2? They are still ranged, which means they won't enter the tile even if they kill the unit, and the target won't fight back. Could buff their ranged attack slightly to compensate for the fact that they make themselves VERY vulnerable in order to enter attack range. It would not require any new mechanics.. just tweaking of a few numbers.