Who switched when, and why it matters
Pope Gregory XIII fixed the calendar in 1582 because the old Julian year of 365.25 days was too long. The calendar had drifted 10 days off the equinox. The reform shortened the average year to 365.2425 days.
Catholic countries adopted it that October. The day after 4 October was 15 October. Ten days vanished.
Britain and its colonies waited until 1752. By then the drift was 11 days. Russia switched in 1918. Greece in 1923. The last major civil switch was Saudi Arabia, which moved from the Islamic calendar for administration in 2016.
A birth recorded in England in 1700 used the Julian calendar. A birth recorded in France the same day used the Gregorian. The dates look different. They describe the same day.
The 11 days Britain deleted
Britain's Calendar (New Style) Act decreed that 2 September 1752 would be followed by 14 September. Eleven days never appeared in British official records.
Calculating the age of someone who died before that September and was born after it means their birth date is Gregorian and their death date is Julian. You cannot subtract the two directly.
The calendar software actually uses
Most software, libraries and online age calculators extend Gregorian rules backwards before 1582, as if the calendar had always existed. The same leap-year rule (divisible by 4, except centuries not divisible by 400) applies to the year 1000 and the year 2000.
This is correct if you were born after your country adopted the Gregorian calendar. It is wrong for historical dates before the switch.
William Shakespeare's birth date is usually given as 23 April 1564. That is the date under the Gregorian rules extended backwards. Under the Julian calendar actually used in England at the time, he was born on 23 April 1564 Julian, which corresponds to 3 May 1564 in the system that projects Gregorian rules backward. The two are not the same.
Does this affect your age calculation today?
No, if you were born after your country's Gregorian adoption date. If you were born in 1990 in the United States, the Gregorian calendar is the only civil calendar that has ever governed your lifetime. The backward-extended Gregorian system used by every major programming language gives you the correct number of days, months and years.
The figures below show how the Gregorian average year translates into lifespan totals.
| Age (years) | Weeks (approx.) | Days (approx.) | Hours (approx.) |
|---|---|---|---|
| 70 | 3,652 | 25,567 | 613,608 |
| 75 | 3,913 | 27,393 | 657,432 |
| 80 | 4,174 | 29,219 | 701,256 |
| 85 | 4,435 | 31,046 | 745,104 |
| 90 | 4,696 | 32,872 | 788,928 |
Each row uses 365.2425 days per year. That is the same constant used to calculate how many days old you are and the same basis for any exact-age tool that counts days lived.
Genealogical age calculations across the switch
Genealogists face a real problem. A family Bible records an ancestor's birth as 10 March 1700 in Massachusetts, still a British colony and still Julian. The same ancestor's death record in 1760 uses the Gregorian calendar because Britain had switched in 1752. Simply subtracting the years gives an age at death that is wrong by 11 days.
The standard approach: convert the Julian birth date to a Gregorian date extended backward first, then compute the difference. For dates before 1700 the offset is 10 days. For 1700 to the country's switch date it is 11 days (1700 was a leap year in the Julian calendar but not in the Gregorian). For Russia, switching in 1918, the offset is 13 days.
If your age calculator uses Gregorian rules extended backward, as most do, enter the Julian birth date converted to Gregorian first. The leap-year complexity is handled automatically, but the tool assumes a Gregorian input. Do the Julian-to-Gregorian conversion yourself.
When the default advice is wrong
Born after 1752 in Britain or after your country's adoption date? Trust the Gregorian age calculator. The numbers in the table above apply to you.
Working with births before the calendar switch? Do not trust the raw dates. Convert them first. Entering the date as-is gives an age that is off by 10, 11 or 13 days depending on the century and country. That matters for a headstone inscription or a family history record.
Why does the Gregorian calendar have 365.2425 days per year?
The Gregorian calendar adds leap years in a pattern that averages 365.2425 days over a 400-year cycle. That is close to the tropical year, the time from equinox to equinox. The Julian calendar used 365.25, which was slightly too long, causing the drift the Gregorian reform corrected.
Does the backward-extended Gregorian calendar change my birthday if I was born after 1752?
No. The backward-extended system matches the Gregorian calendar exactly for all dates after 15 October 1582, or 14 September 1752 for Britain. There is no discrepancy to account for.
How do I convert a Julian birth date to Gregorian for a genealogical age calculation?
For dates between 1582 and 1700, add 10 days. For dates between 1700 and the country's switch date, add 11 days (for Britain, up to 2 September 1752). For Russia and Eastern Orthodox countries switching in the 20th century, add 13 days. After conversion, enter the Gregorian date into any standard age calculator.
Why does the leap-year rule matter for age in days?
The Gregorian rule skips leap years in century years not divisible by 400. Over an 80-year lifespan, that difference amounts to about one full day compared to an "every 4 years" rule. The effect on exact day counts comes from the same 365.2425-day average that produces the table above.