Julian-Gregorian Date Converter

Convert Julian and Gregorian calendar dates with a Julian Day Number

This Julian–Gregorian date converter maps one entered calendar date to its equivalent label in the other calendar. It is useful when reading historical, genealogical, church, astronomical, or archival material in which the same continuous day was recorded under different calendar rules.

How to use the Julian–Gregorian date converter

1. Enter Year, Month (1–12), and Day for the source calendar.
2. Select Julian → Gregorian or Gregorian → Julian to identify the calendar written in the source.
3. Click Convert to display the equivalent date label in the other calendar.

The result includes: the target-calendar date for the same continuous day and the Julian Day Number used by the conversion.

Julian versus Gregorian leap-year rules

The Julian and Gregorian calendars use the same month names and ordinary month lengths, but their leap-year rules differ. That difference is why an historical date can carry two calendar labels.

Because Julian reckoning includes leap days in more century years, Julian date labels gradually move ahead of Gregorian date labels. The difference increases after century years that are not Gregorian leap years, including 1700, 1800, 1900, and 2100.

Julian Day Number method behind the conversion

This Julian–Gregorian converter first turns the entered date into a continuous day index, commonly called a Julian Day Number (JDN), and then reconstructs that same day in the selected target calendar. You do not need to calculate the index manually, but the two-stage method explains why the result preserves the underlying day.

Conceptually, the conversion follows two calendar-specific steps:

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

The conversion relationship can be written as:

Here Y is the year, M is the month from 1 through 12, and D is the day of that month. The selected calendar determines the leap-year correction used when generating the JDN; the inverse calculation returns the date under the other calendar’s rules.

The calculator uses integer calendar algorithms. Gregorian conversion includes the century and 400-year correction, while Julian conversion omits that correction.

Reading the Julian–Gregorian day gap

A Julian–Gregorian result often changes by several days because the calendar gap is historically variable rather than fixed. It changes when the Gregorian calendar omits a century leap day that the Julian calendar retains.

For commonly encountered historical ranges, Julian and Gregorian dates differ by:

• 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: Dates around a century-year February deserve special attention because the difference changes after the calendars apply different leap-year treatment.

Worked example: 1 March 1700 Julian to Gregorian

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

1. The source date is Julian 1700-03-01.
2. By this date, the Julian calendar has included the 1700 leap day that Gregorian rules omit.
3. The Julian label is therefore 11 days earlier than the Gregorian label for the same day.

The converter returns 12 March 1700 (Gregorian). A Julian date before the relevant February boundary can have the earlier 10-day difference instead.

Julian–Gregorian conversion spot checks

These calendar-specific examples can help confirm that the source calendar and conversion direction are selected correctly:

Julian and Gregorian calendar conversion questions

Historical limitations of Julian–Gregorian date conversion

A mathematical Julian–Gregorian conversion identifies the equivalent day in another calendar system, but it cannot by itself establish which calendar a particular place used on that date.

• Regional adoption dates vary: The Gregorian reform began in 1582, but countries and churches adopted it at different times. This converter computes the equivalent Julian or Gregorian date; it does not apply a country-specific cutover automatically.
• Proleptic calendars: Dates outside a region’s adoption period are generally interpreted using proleptic calendars, meaning the Julian or Gregorian rules are extended backward or forward. That mathematical convention can differ from contemporary recordkeeping.
• Cutover discontinuities: Local calendar reforms omitted dates in places that adopted the Gregorian system. The converter maps consecutive days mathematically through a JDN, but it does not identify dates that were skipped in a particular jurisdiction.
• Year numbering (BCE/CE and year 0): The form accepts negative years as BCE values. Historical sources usually move from 1 BCE directly to 1 CE, whereas astronomical numbering includes year 0, so compare BCE results with the numbering convention used by your source.
• Validation matters: The allowed February date depends on the source calendar. A Julian year divisible by 4 is leap, while a Gregorian century year is leap only when divisible by 400.

When Julian–Gregorian records need extra scrutiny

• Dates from the 1500s through the 1900s, when calendar use differed among regions.
• Documents marked “Old Style” (O.S.) or “New Style” (N.S.).
• Transcriptions that may already have converted the original calendar date.

Sources for the calendar conversion method

The converter routes each date through its Julian Day Number using standard Julian and Gregorian calendar algorithms, including formulas presented in Jean Meeus, Astronomical Algorithms (2nd ed., 1998), and the Fliegel–Van Flandern approach referenced by the U.S. Naval Observatory. Historical adoption dates remain contextual information rather than an automatic rule in this calculator.