The Julian + calendar is the predecessor to the Gregorian calendar, originally introduced by Julius Caesar in 46 + BC.
+Julius Caesar (with aid of Sosigenes of Alexandria) introduced this calendar to replace the earlier Roman + calendar, which required manual addition of leap months to keep it in synchrony with the seasons. + Unfortunately, it had leap days every four years without question, resulting in the average year + having 365.25 days. However, the actual tropical year is roughly 365.2422 days, resulting in the Julian + calendar gaining a day every 129 years. This means that seasons drift, starting earlier and earlier in + the year.
+In 1582, Pope Gregory XIII decided to fix this drift by reducing the number of leap days, motivated by a + desire to keep the March equinox on March 21st, since that value was hardcoded in the calculation for + the date of Easter. To achieve this, he made years divisible by 100 but not by 400 non-leap years, + resulting in 97 leap years every 400 years, resulting in the Gregorian calendar. To bring the equinox + back in alignment, October 5th to October 14th in 1582 were deleted. The result is the Gregorian + calendar.
+This version uses the proleptic Gregorian calendar for those dates.
+Since the Gregorian calendar was adopted at different times in different countries, there's no nice way + of handling the transition. In the future, I might make a version that implements the transition with a + country selector, but this version will stay proleptic forever.
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