We are told that Swedenborg was born on Sunday, January 29, 1688. Yet if you take our modern calendar and project backward, you will find that January 29, 1688, fell on a Thursday. How could this be?
In a sense, Swedenborg was a man with two birthdays. From 45 B.C.E. until 1582 C.E. the predominant civil calendar in Europe was the Julian calendar, named after Julius Caesar, who instituted it. The Julian calendar added a leap day every four years. However, because the astronomical year is actually several minutes shorter than 365.25 days, the Julian calendar year was slightly longer than the astronomical year.
By 1582, the difference between the calendar year and the astronomical year was about ten days, which caused an observable difference between the calendar date and the seasons. Pope Gregory XIII corrected this discrepancy by introducing a new system of calculation and adjusting the calendar date forward by ten days. Many Protestant countries initially chose not to adopt the Gregorian calendar because of its Catholic origins, and so between 1582 and 1923 it became important to specify which calendar one was using. A common way to do this was by adding “Old Style” for Julian dates or “New Style” for Gregorian, a system that Swedenborg followed.
Sweden continued using the Julian calendar until 1753, a couple of months after Swedenborg turned 65 years old. By that time, the Swedes had to add eleven days to their calendar to synchronize with the rest of Europe. For that reason, some scholars have suggested that Swedenborg’s actual birthday was February 9, 1688 (New Style).
However, in 1688, the difference between the Gregorian and Julian calendars was only ten days. Therefore, although Swedenborg’s first sixty-five birthdays were celebrated on January 29, Swedenborg was actually born on the day that we now know as February 8.
REFERENCE: Sigstedt, Cyriel Odhner. 1981. _The Swedenborg Epic: The Life and Works of Emanuel Swedenborg._ London: Swedenborg Society.