All I can tell u is the first few lines of a song a local group to me sing.

It was on the 18th day of June Napolean did advance The choicest troops that he could raise Within the bounds of France Thier glittering eagles shone around A terror to their foe But Britain's Lions, tore their wings On the plains of Waterloo.

That would imply that the battle was indeed the 18th of June and the retreat from Russia was during late autumn/winter as snow was one of the factors he was balting against as well as hunger etc.