This episode takes place in three different settings: New Jersey on December 24, 1776, Mount Vernon, Fairfax County, Virginia in 1746 and Pittsburgh, Pennsylvania on December 24, 1892.
The carols being sung at the beginning are "Oh, Come All Ye Faithful" and "Joy to the World." They were composed in 1744 and 1719, both dates before the earliest point of divergence in the timelines Bogg and Jeffrey visit, so they would both exist.