Double birthdays are not typos. When you see a famous person listed with two birth dates, you’re usually looking at the collision of different calendar systems. The most common culprit is the switch from the Julian to the Gregorian calendar, but other regional rules and traditional calendars play a role too.

Why double birthdays exist

Before the modern world converged on the Gregorian calendar, countries used different systems and even different start dates for the new year. When governments reformed their calendars—sometimes centuries apart—dates had to be recalculated (or “converted”) to the new system. That’s how a single birthday can legitimately appear as two dates.

Julian vs Gregorian in 60 seconds

The Julian calendar (introduced by Julius Caesar in 45 BCE) slightly miscalculates the length of the year. Over time it drifts against the seasons. To fix this, Pope Gregory XIII introduced the Gregorian calendar in 1582, skipping a block of days to realign dates with the equinox and tweaking the leap-year rule: century years are leap years only if divisible by 400 (e.g., 1600 and 2000 are leap years; 1700, 1800, 1900 are not).

The result is a growing offset between the two calendars. When countries switched to Gregorian, they “dropped” days and, in some places, reset the start of the legal year to January 1.

Julian–Gregorian offset, at a glance

• 1582–1699: add 10 days to convert Julian → Gregorian
• 1700–1799: add 11 days
• 1800–1899: add 12 days
• 1900–2099: add 13 days
• 2100–2199: add 14 days (effective from Mar 1, 2100)

Note: the offset increases after non-leap century years in the Gregorian system (1700, 1800, 1900, 2100...).

The year-start problem (Old Style vs New Style)

In England and its colonies before 1752, the legal new year began on March 25 (“Lady Day”). Many documents therefore used “Old Style” (O.S.) dating with years that run March 25 to March 24. After reform, the “New Style” (N.S.) year begins on January 1. That means O.S. dates in January–March often convert to a different year in N.S. records. You’ll sometimes see double year notation like 1731/32 to bridge the gap.

Who switched when? A quick map

• 1582: Catholic countries such as Spain, Portugal, Italy, and Poland adopted Gregorian immediately.
• 1582–1700s: Many Protestant regions delayed; adoption dates varied by city-state and kingdom in Germany and the Netherlands.
• 1752: Britain and its American colonies adopted Gregorian, dropping 11 days and moving New Year to January 1.
• 1918: Soviet Russia adopted Gregorian (13-day shift by then).
• 1923: Greece adopted Gregorian (for civil use).
• East Asia: Japan (1873), China (officially 1912 and standardized later), and Turkey (1926 for civil use) moved from traditional or regional calendars to Gregorian.

Classic examples (and how to cite them correctly)

Below are the most quoted “double birthdays” and why both dates appear. For each, the first date is the one used locally at the time; the second is the converted Gregorian date many modern databases standardize to.

William Shakespeare (1564)

Traditional birth date: April 23, 1564 (Julian, England). He was baptized on April 26 O.S.; April 23 is a long-standing tradition often repeated.

Gregorian equivalent: May 3, 1564.

Tip: For consistency, many references keep April 23 as Shakespeare’s celebrated birthday but note “Julian” or “O.S.” and give May 3 as the Gregorian equivalent.

George Washington (1731/32)

Old Style birth date: February 11, 1731 (Julian; British America, with the legal new year beginning March 25).

New Style (Gregorian) date Washington used later: February 22, 1732.

Why both change: The calendar shift adds 11 days, and the new-year shift moves the year from 1731 to 1732.

Isaac Newton (1642/43)

Old Style birth date: December 25, 1642 (Julian).

Gregorian equivalent: January 4, 1643.

Note: Newton’s “Christmas Day” birthday becomes January 4 in the Gregorian system; both dates are commonly cited depending on context.

Benjamin Franklin (1705/06)

Old Style birth date: January 6, 1705 (Julian year; often written 1705/06).

Gregorian equivalent: January 17, 1706.

Practice: U.S. biographies usually give January 17, 1706, the New Style date.

Russian figures (pre-1918 Russia used Julian)

• Vladimir Lenin: April 10, 1870 (Julian) → April 22, 1870 (Gregorian).
• Fyodor Dostoevsky: November 11, 1821 (Julian) → November 23, 1821 (Gregorian).
• Pyotr Tchaikovsky: April 25, 1840 (Julian) → May 7, 1840 (Gregorian).
• Peter the Great: June 9, 1672 (Julian) → June 19, 1672 (Gregorian).

Bonus context: The 1917 “October Revolution” in Russia occurred on October 25 O.S., which is November 7 N.S.—a famous example of how the same event carries two dates.

Sun Yat-sen (1866): lunar vs Gregorian

Gregorian birth date (widely commemorated): November 12, 1866.

Traditional record: In Qing-era China, births were recorded in the lunisolar Chinese calendar. The corresponding lunar date falls in the 11th lunar month of that year, which shifts on the Gregorian calendar from year to year.

How it’s handled: Official commemorations in Taiwan, Macau, and many institutions use November 12 (Gregorian). Some communities also mark his birthday annually by the traditional lunar date, producing a different Gregorian date each year.

Composers in mixed-calendar Europe

• George Frideric Handel: February 23, 1685 (Julian, in a Protestant region still using the Julian calendar) → March 5, 1685 (Gregorian).
• Johann Sebastian Bach: March 21, 1685 (Julian, in a territory that had not yet adopted Gregorian) → March 31, 1685 (Gregorian).

Because German states adopted the Gregorian calendar at different times, musical biographies often list both forms for 17th–18th century birth dates.

Beyond Julian–Gregorian: other systems that create dual dates

East Asian lunisolar calendars

China, Korea, and Vietnam historically used lunisolar calendars where months begin on new moons and leap months are inserted to track the seasons. Converting a historical birth date from the traditional calendar to the Gregorian calendar requires astronomical tables or software. The converted day is fixed for the birth year, but the annual traditional birthday shifts on the Gregorian calendar, just like Lunar New Year moves each year.

Hebrew and Islamic calendars

• Hebrew (lunisolar): Jewish birthdays are often observed on the Hebrew date (e.g., 14 Adar), which falls on different Gregorian dates each year. Historical records may give both.
• Islamic (Hijri, lunar): The Hijri year is about 11 days shorter than the solar year. A birth date like 1 Muharram 1200 AH will correspond to a single Gregorian date in that year, but the anniversary advances earlier each Gregorian year.

In modern biographies, Gregorian dates are usually primary, with traditional dates provided parenthetically.

Simple conversion playbook (Julian ↔ Gregorian)

You don’t need complex math to align most historical birthdays. For dates after 1582, a simple “add X days” approach usually works, plus a check for year-start rules in places like pre-1752 Britain.

Step-by-step

• 1) Identify the original calendar and locale: Was the birth recorded in Julian (e.g., England pre-1752, Russia pre-1918) or already Gregorian (e.g., Spain post-1582)?
• 2) Apply the correct offset for that century: See the offset list above; add 10/11/12/13 days to a Julian date to get the Gregorian date.
• 3) Adjust the year if needed: If the region used March 25 as New Year (England and colonies before 1752), then January 1–March 24 O.S. falls in the previous numbered year. Convert the day, then update the year to the Gregorian system.
• 4) Label clearly: Use O.S./N.S. or “Julian/Gregorian” in parentheses so readers know exactly what each date represents.

Worked examples

• Shakespeare: April 23, 1564 (Julian) + 10 days → May 3, 1564 (Gregorian).
• George Washington: February 11, 1731 (Julian, British America). Add 11 days → February 22, 1731. Then apply New Year shift (Jan–Mar O.S. becomes the next year N.S.) → February 22, 1732 (Gregorian).
• Isaac Newton: December 25, 1642 (Julian) + 10 days → January 4, 1643 (Gregorian).
• Lenin: April 10, 1870 (Julian) + 12 days → April 22, 1870 (Gregorian).
• Benjamin Franklin: January 6, 1705 (O.S., often written 1705/06) + 11 days → January 17, 1706 (Gregorian).

Rule of thumb: For 1700s British/American records, look for both the 11-day shift and the Jan–Mar year adjustment. For 1800s Russian records, apply a 12-day shift; for 1900s before 1918 Russia, use 13 days.

How to present dual dates in publications

• Be explicit: “April 23, 1564 (Julian; traditionally observed), May 3, 1564 (Gregorian).”
• Use double dating where appropriate: For English records between Jan 1 and Mar 24 pre-1752, write “Feb 11, 1731/32 (O.S./N.S.).”
• Choose a house style: Many encyclopedias standardize to Gregorian with the original date in parentheses.
• Mind baptisms vs births: In parish registers, the first date is often a baptism. Shakespeare’s April 26, 1564 is a baptism; the April 23 birthday is inferred and traditional.

Common pitfalls

• Assuming everyone switched at once: They didn’t. Regions changed in different years—even neighboring towns in early-modern Europe could differ.
• Forgetting the year shift: Pre-1752 English dates from Jan–Mar often belong to the next year in Gregorian.
• Conflating calendars: Don’t mix a Julian date with a Gregorian year-number without labeling (e.g., “Feb 11, 1732 O.S.” is inconsistent because O.S. uses the old year numbering).
• Edge cases: Sweden experimented with a unique transition (including a “February 30, 1712”), and the Ottoman Empire used the Rumi civil calendar for a time. For tricky locales, consult specialist tables or software.

Quick resources and methods

• Conversion tables: Many historical almanacs and reputable websites provide Julian↔Gregorian converters with the correct offsets.
• Astronomical approach: For high precision or pre-1582 dates, convert to a Julian Day Number (JDN) and then to the target calendar. Libraries in astronomy and genealogy tools can automate this.
• Primary sources: Parish registers and state papers usually specify O.S./N.S. or imply it from context; always cross-check.

Bottom line

Double birthdays arise because calendars changed. When you see two dates for Shakespeare, Washington, Sun Yat-sen, and many others, you’re seeing a faithful record of historical timekeeping, not an error. With a quick offset, a check of the local new-year rule, and clear labels, you can harmonize “On This Day” entries with confidence.

FAQ

Why do some famous people have two different birthdays?

Because their births were recorded under one calendar (often Julian or a traditional calendar) and later converted to another (usually Gregorian). Some regions also changed the legal start of the year, shifting January–March dates into a different numbered year.

How many days separate the Julian and Gregorian calendars?

It depends on the century: 10 days from 1582–1699, 11 days in the 1700s, 12 days in the 1800s, 13 days in the 1900s and 2000s (until 2099), and 14 days from 2100–2199. The offset increases after century years that are not leap years in the Gregorian system.

Did Shakespeare and Cervantes really die on the same day?

They share the date April 23, 1616 in their local calendars, but not the same day. Spain was using the Gregorian calendar (Cervantes), while England still used Julian (Shakespeare). The calendars were 10 days apart then.

What does 1731/32 mean in old records?

It’s “double dating.” Before 1752 in Britain, the legal year started March 25. A date like February 11, 1731 (O.S.) is February 22, 1732 in the Gregorian system. Writing 1731/32 shows both year numbers depending on the calendar.

How do I convert a Julian birth date to Gregorian quickly?

Identify the century, add the correct number of days (10, 11, 12, or 13), and if the locale used March 25 as New Year, advance the year for dates in January–March. Label the result as Gregorian (N.S.).

Why does Sun Yat-sen have a Gregorian and a lunar birthday?

He was born when China used a lunisolar calendar. His widely commemorated date is November 12, 1866 (Gregorian), while the traditional lunar birthday falls on a date that changes each Gregorian year, similar to how Lunar New Year moves.

Should I standardize to Gregorian in modern writing?

Usually yes, especially for international audiences. Best practice is to give the Gregorian date and include the original calendar date in parentheses with O.S./N.S. labels for clarity.