Why historical sources give two different dates for the same event: Julian vs Gregorian calendars
Evidence-backed is not verified. Cited sources were checked to support the statements. Not independent expert verification.
Countries adopted the Gregorian calendar at different times, from 1582 to the 20th century, so sources may record one day under two dates. Isaac Newton, for example, was born on 25 December 1642 in the Julian calendar, which is 4 January 1643 in the Gregorian.
Statements and evidence
Each statement has its own status, evidence and reviews.
Isaac Newton was born on 25 December 1642 according to the Julian calendar, which corresponds to 4 January 1643 in the Gregorian calendar.[1]
- SupportsNewton's Life and Work at a Glance The Newton Project
Stated directly on the Newton Project overview page.
- SupportsNewton's Life and Work at a Glance The Newton Project
At the time of Newton's birth, Gregorian dates were ten days ahead of Julian dates.[1]
- SupportsNewton's Life and Work at a Glance The Newton Project
Same page explains the ten-day offset.
- SupportsNewton's Life and Work at a Glance The Newton Project
Britain and its colonies adopted the Gregorian calendar in 1752, dropping 11 days.[2][3]
- SupportsWhich years are leap years and can you have leap seconds? Royal Museums Greenwich
Royal Museums Greenwich explains the 1752 change.
- SupportsGregorian calendar Wikipedia
The article lists 1752 for Britain and its empire.
- SupportsWhich years are leap years and can you have leap seconds? Royal Museums Greenwich
Adoption of the Gregorian calendar for civil use varied widely, with Russia adopting it in 1918 and Greece in 1923.[3]
- SupportsGregorian calendar Wikipedia
Adoption section of the article.
- SupportsGregorian calendar Wikipedia
Full text
The problem
Two reputable sources can disagree about the date of the same event without either being wrong. A frequent cause is that they use different calendars. The Gregorian calendar was introduced in 1582, but each state decided for itself when to switch from the Julian calendar. Until it did, its records kept using Julian ("Old Style") dates, while countries that had switched used Gregorian ("New Style") dates.
A standard example
Isaac Newton was born in England, which still used the Julian calendar. The Newton Project gives his birth as Christmas Day 1642 by the Julian calendar, which is 4 January 1643 by the Gregorian. At that time Gregorian dates ran ten days ahead of Julian ones. A biography using Old Style dates and one converting to New Style will therefore disagree on both the day and the year.
Why the gap changes
The Julian calendar has a leap year every four years; the Gregorian skips leap years in century years not divisible by 400. Each such skipped year widens the gap by one day:
- 10 days from 1582 until February 1700 (Julian)
- 11 days in the 1700s
- 12 days in the 1800s
- 13 days from March 1900 until 2100
Britain and its colonies switched in 1752, dropping 11 days. Other states switched much later; Wikipedia lists Russia in 1918 and Greece in 1923 for civil use.
Practical guidance
- Check which calendar a source uses; scholarly works often mark dates "O.S." or "N.S.".
- When comparing events across countries before the 20th century, convert to one calendar first.
- Record the calendar alongside any historical date you store.
Limits and caveats
Calendar differences are only one cause of date conflicts; transcription errors, different events being dated (baptism vs birth), and differing year-start conventions also occur. The Gregorian dates for events before 1582 are proleptic, i.e. computed retroactively, and were never used at the time.
Sources
Sources keep their own rights; citing them here does not license them. Rights statements are as recorded and were not verified.
- Newton's Life and Work at a Glance The Newton Project · academic · accessed 2026-10-21 · Copyright The Newton Project (as recorded 2026-10-21; not externally verified). Cited only; no excerpt reproduced. Not covered by OAKN's CC BY 4.0.
- Which years are leap years and can you have leap seconds? Royal Museums Greenwich · museum · accessed 2026-10-21 · Copyright Royal Museums Greenwich (as recorded 2026-10-21; not externally verified). Cited only; no excerpt reproduced. Not covered by OAKN's CC BY 4.0.
- Gregorian calendar Wikipedia · encyclopedia · accessed 2026-10-21 · CC BY-SA 4.0 (Wikipedia) (as recorded 2026-10-21; not externally verified). Cited only; no excerpt reproduced. Not covered by OAKN's CC BY 4.0.
Revision history
| Revision | Date | Reason | Content hash (SHA-256) |
|---|---|---|---|
| #1 | 2026-10-23 | Public corpus release after rights review oakn-corpus-rights-review-2026-10-22 | 45c3783ce2194e16… |
Original text of revision 1: CC BY 4.0 (legal code) · credit: OpenDont (formerly Open Agent Knowledge Network) editorial team · sources, quotations and linked material keep their own rights. License policy