Genealogy Calendar Conversion

Dates in old records aren't always what they appear. Julian / Gregorian transitions, Quaker numeric months, Hebrew / Hijri / Chinese / Japanese calendar systems, dual-dating across the New Year transition — getting these wrong is one of the most common silent errors in genealogy. For methodology see Genealogy Proof Standard & Sourcing; for translation of foreign-language dates see Genealogy Translation & Paleography.

The transitions to know

• Julian → Gregorian — Catholic Europe in 1582 (jumped 10 days); Britain & colonies in 1752 (jumped 11 days, also moved New Year from March 25 to January 1); Russia in 1918 (jumped 13 days); Greece in 1923. Eastern Orthodox churches still partially use Julian for liturgical dates.
• Old Style / New Style (O.S. / N.S.) — used to distinguish Julian from Gregorian. Dual-dating convention: a date between Jan 1 and March 25 (1582–1752 in British world) often written 1719/20 to mean "1719 by Old Style, 1720 by New Style."
• Lady Day (March 25) — the legal year-start in pre-1752 England. A document dated "5 March 1690" is March 1691 in modern reckoning.

Calendar systems beyond Gregorian

• Hebrew / Jewish calendar — lunisolar; year + month + day notation differs. Civil year vs. religious year. Used in Jewish vital records globally; tombstones often dual-dated.
• Hijri / Islamic calendar — purely lunar; ~354 days per year, so Hijri years and Gregorian years drift apart. Used in many Middle Eastern + South Asian record sets.
• Chinese / Lunar (Lunisolar) — used historically across East Asia; complicates 1900–1950 records.
• Japanese era system (年号 nengō) — years counted within an emperor's reign (Meiji 1, Taishō 5, Shōwa 22, Heisei 14, Reiwa 6). Family registers (戸籍) use these.
• Korean era system — historical use of regnal years until 1894.
• French Revolutionary Calendar — used 1793–1805 in France + briefly in client states. Months named Vendémiaire, Brumaire, etc. Some Napoleonic records use it.
• Quaker plain dating — Quakers wrote month numbers (1st month = March before 1752, January after). "9th day of 7th month, 1670" needs the right rules to decode.
• Saint's-day dating — Catholic records sometimes use feast-day references rather than numeric dates ("Feast of St. John Chrysostom").

Conversion tools

• ★ John Walker's Calendar Converter — free; covers Gregorian, Julian, Hebrew, Hijri, Persian, Mayan, Indian, French Revolutionary, Bahá'í, Chinese. The single most-used calendar-converter page online.
  • https://www.fourmilab.ch/documents/calendar/
• Hebcal — free; Hebrew calendar conversion + holiday lookup; tombstone-date-friendly.
• Quaker Date Calculator (Pennsylvania State Archives) — free; tutorials.
• Family Tree Maker date calculator — paid; built-in.
• RootsMagic date calculator — paid; date arithmetic + conversion.
• Reunion Date Calculator — paid Mac.
• Heredis date utilities — paid French; strong on French Revolutionary calendar.
• Gramps calendar plugin — free; Gramps stores dates with calendar metadata so display can switch between calendars.
• Multi-Calendar Converter (Aldana) — free; Mesoamerican / Aztec / Mayan calendars.

Date arithmetic & age calculations

• Age at death — many tombstones list "X years, Y months, Z days." Reverse-calculate the birth date with day-month-year arithmetic. Don't assume "65 years" rounds up — it might be 64 + 11 months in modern usage.
• GED-style date qualifiers — ABT (about), BEF (before), AFT (after), BET ... AND ... (between), CAL (calculated), EST (estimated). Standard across GEDCOM apps.
• Date gotchas — Feb 30 / Feb 29 in non-leap year recorded; Sept 14 1752 in British colonies (the day after Sept 2 1752, due to skipped 11 days).

Display & storage strategy

• Always store dual when applicable — 1719/20 for the Lady Day window in pre-1752 British records.
• Document the calendar system in source notes — "all dates in this register are Julian Old-Style."
• Standardize for display but preserve original — show modern Gregorian in the tree, but keep "March 5, 1719/20 (O.S.)" in the citation.
• Be aware that Eastern European parish registers may be Julian (Russian Orthodox / Greek-Catholic) until WWI even when surrounding civil records are Gregorian.

Common pitfalls

• Treating "1740" Quaker date as January-1740-month-1 when it's actually March-1740-month-1 (pre-1752 Quaker reckoning).
• Assuming Russian dates 1580–1918 are Gregorian — they're Julian; Russian Revolution dates ("October Revolution" in November Gregorian) are the textbook example.
• Hebrew tombstone dates mistranscribed without converting; Jewish Year 5680 = 1919/1920 Gregorian.
• French Revolutionary dates in Napoleonic records — "16 Brumaire An VII" needs conversion.
• Two-digit years in old transcribed indexes — "23" might be 1823 or 1923.

What's changing in 2024–2026

• AI extraction of dates — Claude / GPT-4o reading old records and converting in one step. Verify; LLMs make calendar errors.
• Wikipedia date templates — improved for non-Gregorian calendars; copy-paste-friendly.
• MyHeritage / Ancestry calendar awareness — modest improvements; still imperfect.

Pick this if…

• Default web converter: John Walker's Calendar Converter.
• Hebrew dates / tombstones: Hebcal.
• Quaker dating: Quaker Date Calculator (PA State Archives).
• Storing dual-calendar dates with metadata: Gramps.
• British pre-1752 ancestor: check Lady Day vs. modern reckoning carefully; dual-date in citations.
• Russian / Ukrainian / Bulgarian pre-1918 records: assume Julian; convert when comparing to Western records.
• French Revolutionary / Napoleonic records: convert via John Walker's tool.