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Convert a Date to a Julian Day

Turn each calendar date into its astronomical Julian Day number (days since 4713 BC). Runs entirely in your browser, so your data never leaves your device.

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Output

The result appears here as you type.

How to use Convert a Date to a Julian Day

  1. 1. Paste your calendar dates. Enter one date per line in YYYY-MM-DD form. Each date is converted independently, so a full list processes in a single pass.
  2. 2. Read the Julian Day number. There are no options to configure. Each date is converted to its astronomical Julian Day number, a continuous day count used widely in astronomy since 4713 BC.
  3. 3. Copy the Julian Day numbers. Copy the resulting numbers into your astronomy script, ephemeris calculation, or dataset wherever a continuous day count is preferred over calendar dates.

When to use Convert a Date to a Julian Day

Convert a Date to a Julian Day turns ordinary calendar dates into the continuous Julian Day number astronomers use to avoid the irregularities of months and leap years. Use it whenever a calculation needs a single, uninterrupted day count.

  • Computing the interval between two dates. You need to know exactly how many days separate two historical events, and converting both to Julian Day numbers turns the problem into a simple subtraction.
  • Feeding an astronomical formula. A formula for calculating a planet's position or a moon phase requires the Julian Day as input, and you have an ordinary calendar date to convert first.
  • Verifying an ephemeris calculation. You are cross-checking a star chart or almanac's stated Julian Day for a given date against an independent calculation before trusting the result.

Examples

The J2000.0 epoch

Input

2000-01-01

Output

2451545

Several dates at once

Input

1970-01-01
2026-07-14

Output

2440588
2461236

About the Convert a Date to a Julian Day tool

Convert a Date to a Julian Day runs as plain JavaScript in your browser tab, with no server behind it. Turn each calendar date into its astronomical Julian Day number (days since 4713 BC). Whatever you put in stays on your device from start to finish.

The tool is part of EditSafely's Time Tools section, 90 single-purpose utilities built around the same idea: open the page, get the result, keep your data to yourself.

There is nothing to configure. Provide the input and the result appears on its own. 2 worked examples further down the page show exactly what the tool produces for real inputs.

That local-first design has practical benefits beyond privacy. The tool keeps working on a flaky connection once the page has loaded, results are instant because nothing round-trips to a server, and it is safe to use with confidential material.

Frequently asked questions

Does Convert a Date to a Julian Day cost anything?

Yes, it is completely free. All 2,658 tools on EditSafely work without an account, a subscription or usage limits.

Is it safe to paste sensitive or confidential data?

No data leaves your device. The whole tool is JavaScript that runs inside your browser tab, so there is no upload, no server-side processing and no log of what you did. If you disconnect from the internet after the page loads, it keeps working.

How much text can I process at once?

There is no fixed limit. Because the work happens on your own device rather than on a shared server, the practical ceiling is your machine's memory, which comfortably handles inputs far larger than typical online tools allow.

Do I need to sign up or install anything?

Nothing to install and no account needed. Open the page in any up-to-date browser, including on a phone or tablet, and the tool is ready.

How do I use the result?

The output panel has a one-click copy button, and you can keep refining the input while you work; the result updates in place as you type.

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