How to Convert a Date to Gregorian Value
The ancient Julian calendar had leap years every four years, since the time it takes the Earth to go around the sun, or "tropical year," is not precisely 365 days. However, tropical year is less than 365.25 days. Therefore, over the course of centuries, the Julian calendar of antiquity came to trail the seasons by several days. Pope Gregory addressed this by modifying the leap year system in 1582, taking away the "leap day" from years divisible by 100. Years divisible by 400 retained their extra day. Since the introduction of the Gregorian calendar, the difference between the Gregorian and Julian calendar has increased three days every four centuries. Between years 1900 and 2100, the difference between the two calendars stands at 13 days. Conversion from a Julian date to a Gregorian date is a matter of simple arithmetic, once you have the formula.
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Instructions
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Truncate the digits to the right of the hundreds place in the year in question. For example, for the year 1500, just look at the 15.
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Multiply the result of Step 1 by 0.75.
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Subtract 1.25.
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Truncate any decimal points. This result is how many days to add to the Julian date that you are trying to convert to a Gregorian value. For example, December 1, 1215, has the calculation 12x.75-1.25 = 7.75. Truncating gives 7 days. So a Julian date of December 1, 1215, is December 8, 1215.
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Handle BC dates the same way but subtract a year before doing any calculations. Then add the year back in at the end. The reason is to maintain the linear relationship of the formula, given that there is no 0 BC or 0 AD. 1BC was followed the next year by 1AD.
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Tips & Warnings
According to Wolfram Research, the current Russian and Persian calendars follow the seasons even more accurately than the Gregorian calendar.
In years divisible by 100, the calculations for the first few days in March become more problematic. You may want to refer to an online calculator for the rare instances (less than 100 days in the past two millennia) when you need to make this calculation.
References
Resources
- Photo Credit calendar image by Aleksey Bakaleev from Fotolia.com
