The Julian Calendar was a calendar designed by the Alexandrian astronomer Sosigenes in 46 BC and enforced throughout the Roman Empire by Julius Caesar in 45 BC. It remained in use throughout the Western world until 1582, when inaccuracies in the Julian calendar were found to have created a lag of ten full days in the Julian calendar; for instance, the vernal equinox was marked ten days after the day which had twelve hours of sunlight and twelve hours of darkness. A revised calendar correcting this problem, the Gregorian Calendar, was proposed to and decreed by Pope Gregory XIII. The Julian Calendar was abandoned in Catholic countries but remained in use in Protestant countries for centuries. Many Eastern Orthodox countries use either the Julian calendar or a variation, primarily for calculating the dates of feasts.
The Julian calendar was still in use throughout most of Protestant Europe when Grantville arrived in the year 1631. However, in the world Grantville had left, the Gregorian calendar was the universal calendar. Thus, when Michael Stearns declared that there would be a celebration on July 4, 1631, certain of the town's allies briefly raised an eyebrow, but the issue did not cause any major conflict.
It should be noted that not all areas which used the Julian calendar in the 1630s started their civil or numbered year on January 1. In general, Western Europe had made this change by 1631, as had Scotland. England, however, still started its year in March.