Julian Date Converter

Convert a calendar date and time to Julian Date (JD), or convert JD back to a date, with MiniWebtool’s Julian Date Converter. View JDN and MJD alongside the day-of-year number.

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Help me understand MiniWebtool's Julian Date Converter: https://miniwebtool.com/julian-date-converter/. What can I use it for, what inputs or settings do I need, and how do I use it and understand the output? Walk me through one simple example. Use this page as your source and answer in English.

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2000-01-01 at 12:00 UT is Julian Date 2451545.0

Convert a calendar date and time to a Julian Date, or a Julian Date back to a date. The result also shows the JDN, the MJD and the day of the year.

Quick examples — click to fill the form, then press Convert:

Year

Month

Day

Time of day (Universal Time)

Hour

Minute

Second

Calendar

Embed Julian Date Converter Widget

🔭 Julian Date Wednesday

2461348.50000

November 4, 2026 · 00:00:00 UT (Gregorian calendar)

Calculation model: 86,400 seconds per day, without leap seconds or UTC/UT1/TT conversion. JD − 2451545.0 is a numerical offset here, not a precision elapsed-time measurement. Displayed decimal places do not establish timing accuracy.

JDN 2461349 MJD 61348.00000 Day of year 308

🧮 Every Julian / Date Format

Julian Date (JD)

2461348.50000

days since 4713 BC, noon UT

Julian Day Number (JDN)

2461349

integer day count

Modified Julian Date (MJD)

61348.00000

JD − 2400000.5 · day starts midnight

Days since JD 2451545.0

9803.50000

JD − 2451545.0

Unix timestamp

1793750400

seconds since 1970-01-01 UTC

Year + day of year

2026308

Gregorian · YYYYDDD

Reduced JD (RJD)

61348.5000

JD − 2400000

Truncated JD (TJD)

21348.00000

JD − 2440000.5 (NASA)

📏 Where Your Date Sits in Time

Cosmic scale — the full Julian Day count from its 4713 BC origin to today

4713 BC · JD 0

AD 1

JD 2451545.0

Your date

Zoomed in — your date next to the JD 2451545.0 epoch and today (1995 → 2031)

JD 2451545.0

Today

Your date

🕛 The Day Fraction & Noon Boundary

Unlike the civil day, a Julian Day rolls over at noon Universal Time (the red mark), not at midnight. The blue hand shows your time of day, 00:00:00 UT.

That is why the decimal part of your Julian Date is:

0.50000 of a day past noon

A civil midnight therefore lands on a clean .5 fraction.

📝 Step-by-Step Conversion

Step 1: Set the working year and month

The month is March or later, so the year and month are used as-is:

$$Y = 2026, \quad M = 11$$

Step 2: Gregorian calendar correction

$$A = \left\lfloor \frac{Y}{100} \right\rfloor = 20$$$$B = 2 - A + \left\lfloor \frac{A}{4} \right\rfloor = -13$$

Step 3: Apply the Julian Date formula

$$JD = \lfloor 365.25(Y+4716) \rfloor + \lfloor 30.6001(M+1) \rfloor + D + B - 1524.5$$$$JD = 2462515 + 367 + 4.00000 + -13 - 1524.5 = \mathbf{2461348.50000}$$

D includes the fraction of the day from the time of day (the Julian Day starts at noon UT, so 00:00 UT carries a 0.5 fraction).

Algorithm after Jean Meeus, Astronomical Algorithms, chapter 7. Input is read as Gregorian calendar; the Gregorian equivalent of this instant is November 4, 2026.

Video Guide: Julian Date Converter

Video guide with full transcript

About Julian Date Converter

A day-of-year number runs from 1 to 365 or 366. Astronomical Julian Date (JD) is a continuous day count with a time fraction. Day of Year Calendar

The Julian Date Converter turns any calendar date into its Julian Date (JD) and Julian Day Number (JDN) — and converts straight back the other way. It also gives you the Modified Julian Date (MJD), days since the JD 2451545.0 epoch, the Unix timestamp, the day of the year, the weekday, and the 5-digit ordinal "Julian date" used in industry — all from a single conversion, with a visual time-scale and a step-by-step formula.

J2000.0 is defined at JD 2451545.0 TT. This calculator does not convert its inputs to TT. USNO: Terrestrial Time

At a Glance

Julian Date Converter turns a calendar date and time into a Julian Date (JD) and Julian Day Number, and converts a JD, Modified JD or Unix timestamp back to a calendar date. It shows related date formats beside the result.

Julian Date Converter: features at a glance
Feature	What you get	Good for
Date to Julian	Enter year, month, day, hour, minute and second, and choose the Gregorian or Julian calendar.	Dating astronomical observations or historical events
Julian to date	Enter a number and choose its type: Julian Date, Modified Julian Date or Unix timestamp in seconds. The result is a calendar date.	Reading dates from ephemerides, satellite data or logs
Related day counts	Shows JDN, MJD, Reduced JD, Truncated JD, Dublin JD and days since JD 2451545.0, plus the Unix timestamp, day of year and weekday.	Converting between the JD variants used in different fields
5-digit ordinal date	Also shows the ordinal date used in manufacturing and food packaging, such as 26180 for day 180 of 2026.	Reading date codes on packaging or mainframe records
1582 calendar reform	When converting a JD back to a date, results are Gregorian on or after October 15, 1582, and Julian before that date.	Historical dates around the calendar change
Formula steps	Displays the step-by-step calculation, a cosmic time-scale and a noon-boundary 24-hour dial.	Learning how Julian Dates are computed

What Is a Julian Date?

A Julian Date is a continuous count of days, including the fraction of a day, measured from noon Universal Time on January 1, 4713 BC (in the proleptic Julian calendar). Because it ignores months, varying month lengths, leap years and calendar reforms, it makes date arithmetic trivial: the number of days between two events is simply one Julian Date subtracted from another. Astronomers, satellite operators and historians rely on it for exactly this reason.

Julian Date vs. Julian Day Number

These two terms are often mixed up:

Julian Day Number (JDN) — the whole-number day count, with no time of day.
Julian Date (JD) — the JDN plus the fraction of the day for the time. Since the day begins at noon, midnight carries a .5 fraction.

The Julian Date Formula

For a date in the Gregorian calendar, with the time folded into a fractional day D, the Julian Date is computed in three short steps.

Step 1 — Adjust the month

$$\text{If } M \le 2:\quad Y \leftarrow Y - 1,\quad M \leftarrow M + 12$$

Step 2 — Gregorian correction

$$A = \left\lfloor \frac{Y}{100} \right\rfloor, \quad B = 2 - A + \left\lfloor \frac{A}{4} \right\rfloor$$

Step 3 — Julian Date

$$JD = \lfloor 365.25(Y+4716) \rfloor + \lfloor 30.6001(M+1) \rfloor + D + B - 1524.5$$

For the Julian calendar, the correction term is simply B = 0. To go the other way — from a Julian Date back to a calendar date — the tool applies Meeus' inverse algorithm, which automatically returns Gregorian dates on or after October 15, 1582 and Julian dates before it.

Related Julian Date Variants

Julian Date Converter: Related Julian Date Variants
Name	Definition	Day starts	Typical use
Julian Date (JD)	—	Noon	Astronomy
Modified JD (MJD)	JD − 2400000.5	Midnight	Satellites, computing
Reduced JD (RJD)	JD − 2400000	Noon	Variable-star work
Truncated JD (TJD)	JD − 2440000.5	Midnight	NASA missions
Dublin JD (DJD)	JD − 2415020	Noon	Older ephemerides
Days since JD 2451545.0	JD − 2451545.0	Noon	Modern astronomy

The 5-Digit "Julian Date" Is Different

In manufacturing, food packaging, military logistics and mainframe systems, a "Julian date" usually means a 5-digit ordinal date such as 26180, made of the last two digits of the year followed by the day of the year (day 180 of 2026). It has nothing to do with the astronomical Julian Day count. To avoid any confusion, this converter shows both the true Julian Date and the ordinal date for every result.

Worked Example

Take noon UT on January 1, 2000 — the famous JD 2451545.0 epoch:

Month is January, so set Y = 1999, M = 13.
Gregorian correction: A = 19, B = −13.
JD = 2452653 + 428 + 1.5 − 13 − 1524.5 = 2451545.0, with JDN 2451545.

How to Use This Calculator

Choose a direction: "Date → Julian" to convert a calendar date, or "Julian → Date" to convert a JD, MJD or Unix value.
Enter your values: For a date, fill in the year, month, day and time and pick the Gregorian or Julian calendar. For a Julian input, type the number and select its type.
Click Convert: The astronomical conversion runs instantly.
Read the results: See the Julian Date and every related format, copy any value with one click, locate your date on the time-scale and 24-hour dial, and follow the step-by-step formula.

Frequently Asked Questions

What is a Julian Date?

A Julian Date (JD) is a continuous count of days and fractions of a day since noon Universal Time on January 1, 4713 BC in the proleptic Julian calendar. Astronomers use it because it removes months, leap years and calendar reforms from date arithmetic, so the difference between two events is simply a subtraction.

What is the difference between Julian Date and Julian Day Number?

The Julian Day Number (JDN) is the integer day count, while the Julian Date (JD) includes the fraction of the day for the time. For example, noon on January 1, 2000 has JDN 2451545 and JD 2451545.0, while midnight that morning has JD 2451544.5.

What is the Modified Julian Date (MJD)?

The Modified Julian Date is MJD = JD − 2400000.5. It uses smaller numbers and, unlike the Julian Date, its day starts at midnight rather than noon, which makes it convenient for satellite tracking and many computing applications.

Why does the Julian Day start at noon?

The Julian Day begins at noon Universal Time so that a single night of astronomical observation falls within one Julian Day rather than being split across a midnight date change. This is why a civil midnight carries a 0.5 day fraction.

Is the 5-digit Julian date the same thing?

No. The 5-digit ordinal date used in manufacturing, food packaging and mainframes, such as 26180, is the last two digits of the year followed by the day of the year. It is sometimes called a Julian date but is unrelated to the astronomical Julian Day count. This converter shows both so you are never confused.

Which calendar should I choose?

Use Gregorian for modern dates and most purposes. Choose Julian only for historical dates before the 1582 reform when your source explicitly uses the old-style Julian calendar. When converting a Julian Date back to a date, this tool automatically returns Gregorian for dates on or after October 15, 1582 and Julian before it.

Additional Resources

Julian Day - Wikipedia

Reference this content, page, or tool as:

"Julian Date Converter" at https://MiniWebtool.com/julian-date-converter/ from MiniWebtool, https://MiniWebtool.com/

by miniwebtool team. Updated: June 29, 2026

Developer API available: Run this tool from your app, automation, or agent with one JSON HTTP request. View API docs

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