Julian to Gregorian calendar date converter

Free online converter for old-style (Julian) and new-style (Gregorian) dates. Automatically computes the era-dependent offset (10 days before 1700, 11 before 1800, 12 before 1900, 13 in the 20–21st centuries) using the classic Julian Day Number algorithm. Shows weekday, JDN, ISO format, fixed Orthodox feasts in new style, and Easter (Orthodox vs Catholic).

How to use

1
Enter a date

In either field — old or new style. The other updates automatically.

2
Check the weekday

Both styles share the same weekday for the same day. October 25 (OS) 1917 = November 7 (NS) 1917, same Wednesday.

3
Read the feasts

Scroll for the Orthodox fixed feasts and Easter dates for the chosen year.

Convert dates between Julian and Gregorian calendars in seconds

📜 Old Style
Julian calendar
—
⇄
📅 New Style
Gregorian calendar
—
—
Difference
—
JDN
—
ISO 8601
ℹ️ Gregorian calendar introduced October 15, 1582 (right after October 4 Julian). Russia adopted it February 14, 1918. Pre-1582 conversion is "proleptic" — for historical reference only.
Difference by era
✝️ Fixed Orthodox feasts
Date on Julian calendar, observed on the corresponding new-style date for the chosen year.
🕊️ Easter

Features

Precise JDN algorithm Era-dependent offset Bidirectional input Orthodox feasts Easter Share

FAQ

Why two calendars?

The Julian calendar (Julius Caesar, 45 BC) drifted ~10 days from the solar year over 1500 years. Pope Gregory XIII reformed it in 1582 — October 4 was followed directly by October 15. Russia adopted Gregorian only on February 14, 1918.

Why does the difference grow?

Julian adds a leap year every 4 years. Gregorian skips century years not divisible by 400 (1700, 1800, 1900 — not leap; 2000 — leap). Each skipped year shifts the gap by 1 day.

Which style for genealogy?

Russian Empire records before 1918 use old style. Modern publications often print dates as "October 25 (November 7) 1917" — old (new). Check the source.

When will calendars align?

Never — the gap only grows. By 2100 it will be 14 days, by 2200 — 15. In 43,000 years a whole year accumulates.

What is the Julian Day?

JDN — day count from January 1, 4713 BC. Used by astronomers and by this tool for exact conversion. Today JDN ≈ 2,460,000.

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We can — and it's free! Just send us a quick message with your idea. If you'd like to discuss it in detail, leave your email and we'll get back to you. You can stay anonymous.

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Who is this tool for?
📚 Educators & Teachers
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