Julian-Gregorian Date Converter

Convert Julian ↔ Gregorian dates (and what the result means)

This calculator converts a single calendar date between the Julian calendar and the Gregorian calendar. It’s commonly used in history, genealogy, church records, astronomy logs, and archival research where the “same day” may be written differently depending on which calendar a region was using at the time.

How to use

1. Enter Year, Month (1–12), and Day (valid for that month in the selected source calendar).
2. Choose a direction: Julian → Gregorian or Gregorian → Julian.
3. Click Convert to see the equivalent date in the other calendar system.

What you’ll get: the corresponding date in the target calendar for the same continuous day count (via a Julian Day Number-style day index under the hood).

Julian vs Gregorian: what’s different?

Both calendars have the same months and mostly the same day counts, but they differ in how they define leap years. That difference causes the calendars to drift apart over centuries.

Because the Julian calendar treats more years as leap years, it gradually “runs ahead” of the Gregorian calendar. The gap between them increases stepwise around century years that are not Gregorian leap years (e.g., 1700, 1800, 1900, 2100).

The core idea (formulas): convert via an absolute day count

Most robust conversion methods work by mapping a calendar date to a continuous day index (often called a Julian Day Number, JDN), then mapping that day index back into the other calendar. You don’t need to do this by hand—the calculator does it instantly—but knowing the approach helps you interpret results.

Conceptually:

• Step 1: (Y, M, D) in the source calendar → JDN
• Step 2: JDN → (Y, M, D) in the target calendar

A common way to describe this is:

Where Y is year, M is month (1–12), D is day (1–31), and calendar determines which leap-year rule is applied. The inverse function takes the JDN and returns the date in the desired calendar.

In practice, published algorithms use integer arithmetic with terms like “century” and “leap-year corrections.” The key difference between Julian and Gregorian conversion to/from JDN is the presence (Gregorian) or absence (Julian) of the century/400-year correction.

Interpreting the result (and the day-gap you’ll see)

If you convert a date and notice the output is shifted by some number of days, that’s expected. The difference between calendars is not constant across all centuries; it changes when Gregorian rules skip a leap day that the Julian system would have included.

Typical Julian–Gregorian offsets for many common historical ranges:

• 1582-10-15 to 1699-12-31: 10 days
• 1700-03-01 to 1799-02-28: 11 days
• 1800-03-01 to 1899-02-28: 12 days
• 1900-03-01 to 2099-02-28: 13 days

Note: around the century boundary itself, the effective offset can depend on whether you are before/after the leap-day in that year.

Worked example

Example: Convert 1 March 1700 (Julian) to Gregorian.

1. Source calendar: Julian
2. Date: 1700-03-01
3. In the 1700s, the typical offset after 1700-03-01 is 11 days.

So the corresponding Gregorian date is typically 12 March 1700 (Gregorian). (If you try a date just before the century leap-day boundary, you may observe a 10-day vs 11-day offset depending on the exact day.)

Quick confidence-check examples

These are handy spot-checks when you want to verify you selected the correct conversion direction:

Limitations & assumptions (read this for historical accuracy)

Calendar conversion is trickier than it looks because “which calendar was used” depended on place and time, not just the date itself.

• Regional adoption dates vary: The Gregorian reform began in 1582, but countries and churches adopted it at different times (for example, Great Britain and its colonies in 1752; Russia in 1918; Greece in 1923/1924 depending on context). This converter computes the mathematical equivalent date between systems; it does not automatically apply a country-specific cutover rule.
• Proleptic calendars: For dates outside the historical adoption period, results are typically interpreted using the proleptic Gregorian or proleptic Julian calendar (extending rules backward/forward in time). That can differ from how contemporaries recorded dates.
• Cutover discontinuities: In places that adopted the Gregorian calendar, there were “skipped” local dates (e.g., the well-known October 1582 adjustment in some regions). A purely mathematical conversion will still map day-by-day via an absolute count, but it won’t tell you which dates were omitted locally.
• Year numbering (BCE/CE and year 0): Historical calendars usually go from 1 BCE directly to 1 CE (no year 0). Many astronomical algorithms use astronomical year numbering that includes year 0. Unless the calculator explicitly documents BCE support, assume inputs are intended for CE/AD years as positive integers. If you need BCE handling, confirm whether the tool expects astronomical numbering.
• Validation matters: Month/day validity depends on the source calendar’s leap-year rule. For example, February 29 exists in a Julian leap year whenever the year is divisible by 4; in the Gregorian calendar, century years require divisibility by 400.

When you should be extra careful

• Dates in the 1500s–1900s when different regions used different calendars simultaneously.
• Records labeled “Old Style” (O.S.) vs “New Style” (N.S.).
• Transcriptions that already applied a conversion without stating it.