Gregorian date ⇄ Julian Day Number (JDN), fractional JD, Modified Julian Date (MJD) and ordinal day-of-year — astronomy-grade, browser-only
- Runs locally
- Category Format Converter
- Best for Turning pasted content or local files into a handoff-friendly format.
What this tool does
Convert any Gregorian calendar date to its astronomical Julian Day Number and back. The Julian Day Number is the continuous count of days astronomers use to timestamp observations without calendar headaches: day zero is noon Universal Time on January 1, 4713 BC. Enter a year, month, day and the time of day in UT; the tool returns the integer JDN (taken at noon), the fractional Julian Date JD where midnight falls on the .5 boundary, and the Modified Julian Date MJD = JD minus 2400000.5 whose epoch is 1858-11-17 and which rolls over at midnight. It also shows the ordinal day-of-year, the thing IT and mainframe people call a Julian date, and explains why that is a completely different number. The math uses the Fliegel and Van Flandern integer algorithm on the proleptic Gregorian calendar, so it stays exact for ancient and far-future years that break the JavaScript Date object. One-click copy and a shareable link reproduce your exact conversion. Runs entirely in your browser, nothing is uploaded.
Tool details
- Input
- Numbers
- The page exposes text boxes, numeric controls, file pickers, or structured inputs depending on the tool.
- Output
- Live result + Copy
- The result area focuses on usable output, with copy, download, or preview actions when supported.
- Privacy
- Browser-side processing
- The main tool logic does not call an external API, so inputs normally stay in the current tab.
- Save / share
- No account required
- Open the page and use it; whether results survive refresh depends on the tool.
- Performance budget
- Initial JS <= 9 KB
- No WASM budget is declared, keeping the tool quick to open on mobile.
- Best fit
- Format Converter · Student
- Category and role tags drive related tools, internal links, and quick fit checks.
How to use
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1. Input
Paste or drop your content into the tool panel.
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2. Process
Click the button. All processing is local in your browser.
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3. Copy / Download
Copy the result or download to disk in one click.
How Julian Date Converter fits into your work
Use it when the main problem is getting content from one practical format into another.
Conversion jobs
- Turning pasted content or local files into a handoff-friendly format.
- Previewing a conversion before you use it in a larger workflow.
- Cleaning small format mismatches without opening a full editor.
Conversion checks
- Try a small sample first when the source format is messy.
- Check character encoding, separators, and line endings after conversion.
- Keep the source until the converted output has been reviewed.
Good next steps
These links move the current task into a more complete workflow.
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Real-world use cases
Timestamp an astronomical observation
You logged a variable-star brightness at 21:30 UT and need a single number your reduction software accepts. Enter the date and time and the tool gives the fractional Julian Date, the field every photometry and spectroscopy pipeline expects. No more hand-converting a calendar date into a JD and second-guessing whether you got the noon boundary right.
Compute the gap between two dated events
How many days separate the launch of a probe and a flyby years later? Convert each calendar date to its JDN and subtract. Because the count is continuous, the answer drops out as plain arithmetic with no leap-year bookkeeping, even when the two dates straddle a century boundary like 2000 that breaks naive day counting.
Read or write Modified Julian Dates in data files
A satellite telemetry file or a radio-astronomy header stamps every row with an MJD like 60810. Type that day back into the tool, or go the other way and turn a calendar date into the MJD a logging format expects. The JD and MJD sit next to each other so you can confirm the 2400000.5 offset at a glance.
Teach or learn how calendar systems line up
An astronomy or history class needs to see that the Gregorian, ordinal, and Julian Day numbers describe the same instant in different units. Put in a famous date, watch the JDN, the day-of-year, and the MJD appear together, and the relationship between continuous counting and our month-based calendar becomes concrete instead of abstract.
Common pitfalls
Mixing up the astronomical Julian Day with the IT day-of-year. A value like 2024061 is the 61st day of 2024, not a JDN in the millions. If your number resets to 1 each January, it is the ordinal date, not the continuous Julian count.
Forgetting the day rolls over at noon. Midnight is the .5 boundary, so 2000-01-01 00:00 is JD 2451544.5, not 2451545.0. Treating midnight as the integer point shifts everything by half a day and quietly corrupts time differences.
Dropping or adding the 2400000.5 when moving between JD and MJD. MJD is JD minus exactly 2400000.5, including the half. Rounding it to 2400000 puts you twelve hours off and lands every converted date on the wrong calendar day.
Privacy
Every conversion runs as plain JavaScript inside your browser tab. The date, time, and the Julian numbers never leave the page, and nothing is logged. One caveat: the shareable link encodes your date and time in the query string, so a link pasted into chat records those values in the recipient server access log. If the date is sensitive, use the copy button and paste the text instead of sharing the URL.
FAQ
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