When we cut away to Peter's segment, we are told it is 1670. Later, when Peter gets divorced, we're told it is 18th century justice; that should have been 17th century justice. However, a split second after the title card "1670" appears, another line appears saying "or 1760, whatever." This indicates it's either the 17th or 18th Century, both are true.
When Stewie is riding down the road, he says "Let's drive on the left side of the road and see if that catches on", but they were already on the left side of the road.