It's just a goof, or a deliberate disregard to reality in order to make a fun movie. Here's a few relevant facts from the goof section on IMDB
Ben, Abigail and Riley follow a clue from the engraving of Independence Hall on the $100 bill. Noting the time on the clock in the tower and following the shadow of the tower to a spot on a wall. In reality the original tower of Independence Hall that existed in 1776 had no clock, it was built only to hold a bell (i.e. the Liberty Bell.) What's more that tower became unstable and was removed in 1781 and was replaced by a simple peaked roof. The original clock on the building was built into the side of the building facing Congress Hall. The current tower with its clock wasn't built until 1828. Benjamin Franklin who was the focus of many of the clues died in 1790 so there's no way he could have used the tower clock in setting up the clues. This information comes from "Historic Landmarks of Philadelphia" by Roger W. Moss.
Until 1883, there were no standard time zones. Almost every locality used solar time to determine the correct time (noon was the time when the sun was directly overhead in each city, and this, of course, varied depending on the location of the city). Standard time zones were established when railroad schedules necessitated them. The only way they could have determined when 2:22PM occurred in the 1700s was to find out when the sun was directly overhead, and add two hours and twenty-two minutes to that time.