Julian Date Converter

Convert UTC calendar dates into JD, MJD, JDN, J2000 days, ordinal day, and copy-ready scientific date formats.

100% Browser-Based•Local Processing

Gregorian UTC Inputcustom controls

Signed years are allowed.

Month

Valid UTC date.

Calendar rules

Controls JD and MJD decimal places.

Scientific Outputslive results

Julian Date (JD)--
Modified Julian Date (MJD)--
Julian Day Number (JDN)--
Days from J2000--
Ordinal date--
ISO UTC--

Reverse Converter

Value type

Enter JD, MJD, or JDN to update the calendar input.

Bulk Conversionone per line

Accepted line format: YYYY-MM-DD HH:MM:SS. Bulk conversion stays in your browser.

Julian Date Converter workspace with Gregorian UTC input and live JD MJD JDN outputs
Use the custom UTC inputs, calendar rule selector, and precision slider to generate scientific Julian date outputs.
Reverse Julian Date conversion from JD value to Gregorian UTC calendar date
The reverse converter helps turn JD, MJD, or JDN values from papers, logs, and datasets back into a calendar date.
Julian Date converter shown on an astronomy workstation for scientific time conversion
The edited observatory-style image uses a real UI screenshot composited into a research workspace.
Unix Timestamp Date to Timestamp Timestamp to Date ISO Week Date Day of Week Military Time

Privacy Focused

🔒 Local Processing. Your data never leaves your device.

Instant Results

🌐 Fully Client-Side. Runs instantly in your browser.

No Signup

⚡ No accounts. No hassle. Just open and use.

Browser Based

🚀 Works right in your browser. No installs, no downloads.

Overview

Convert Calendar Dates into Scientific Day Counts

Julian Date Converter turns a UTC calendar date and time into Julian Date (JD), Modified Julian Date (MJD), Julian Day Number (JDN), J2000 days, ordinal day, and ISO UTC output. It also reverses JD, MJD, or JDN values back into a readable calendar date.

Julian Date conversion is common in astronomy, satellite tracking, scientific datasets, time-series engineering, and historical chronology. The hard part is not typing a date; it is knowing which value you need. JD includes the fractional day, JDN is the whole day count, MJD is a shorter modern variant, and ordinal day is the day-of-year format many non-astronomy users call a Julian date.

This tool keeps those outputs side by side so you do not confuse them. Use it with the Unix Timestamp Converter when comparing server logs, the ISO Week Date Converter for week-based dates, or the Day of Week Calculator when you need a human calendar check.

Astronomy

Convert observation times into JD, MJD, JDN, and J2000 day offsets.

Data Work

Normalize timestamps for logs, datasets, and copy-ready scientific tables.

Operations

Check ordinal date and CYYDDD codes used in manufacturing and inventory systems.

Guide

How to Use the Julian Date Converter

Enter the UTC calendar date and time

Set the year, month, day, hour, minute, and second in the input panel. The converter uses UTC-style civil time for the form fields, then applies the Julian Date noon offset internally so you do not need to adjust the time manually.

Choose calendar rules and precision

Use Gregorian calendar rules for modern civil dates. Use Julian calendar rules when you are matching a historical source that explicitly uses the older Julian calendar. Increase precision when you need fractional-day values for science or code.

Copy the output or reverse a Julian value

Copy JD, MJD, JDN, J2000, ordinal day, or ISO UTC from the result cards. If you already have a JD-like value from a paper or dataset, paste it into the reverse converter and choose whether it is JD, MJD, or JDN.

Support

Frequently Asked Questions

What is a Julian Date?

A Julian Date is a continuous count of days used in astronomy and scientific timekeeping. The fractional part represents the time of day. Unlike normal calendar dates, JD avoids month lengths and leap-year boundaries when comparing moments.

What is the difference between JD, JDN, and MJD?

JD is the full fractional Julian Date. JDN is the whole day number. MJD is JD minus 2400000.5, which makes the number shorter and aligns the day boundary with midnight for many modern records.

Why does Julian Date start at noon?

Julian Date starts at noon because astronomers historically wanted one observing night to stay under the same day number. The converter accepts normal UTC time and handles that noon boundary automatically in the JD output.

Can I convert a Julian Date back to a calendar date?

Yes. Enter the value in the reverse converter and choose JD, MJD, or JDN. The Gregorian UTC input updates to the matching calendar date and time, which is useful when reading scientific papers or log files.

Is Julian Date the same as day-of-year?

No. Astronomy uses Julian Date as a continuous day count. Some industries use "Julian date" to mean day-of-year, such as 2026-153. This converter shows ordinal day output separately to prevent that common mix-up.

What is J2000 in Julian Date calculations?

J2000 is the reference epoch JD 2451545.0, which corresponds to 2000-01-01 12:00 UTC. The J2000 output shows how many days the selected moment is before or after that epoch.

Does this converter upload my dates?

No. The conversion runs locally in your browser. Your calendar inputs, Julian values, bulk list, and copied outputs are not uploaded to Randomly.online or processed by a server.

Can I use negative years or BCE dates?

Yes. The year field accepts signed astronomical year numbering. Use caution when matching historical sources because calendar rules and Gregorian adoption dates varied by country and record system.

Still have questions?

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