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

The Julian Day Number (JDN) is an integer representing the day count starting at noon. The Julian Date (JD) is a decimal number that includes the time of day as a fractional part. For example, JDN 2451545 corresponds to January 1, 2000, while JD 2451545.5 corresponds to midnight that day.

How do I convert a calendar date to Julian Date?

Enter a date in YYYY-MM-DD format (optionally with time HH:MM:SS) into the converter above, select 'Calendar → Julian Date' mode, and click Convert. The tool calculates the Julian Day Number and optionally the Modified Julian Date and Julian Ephemeris Date instantly.

What date formats are supported?

Supported input formats include YYYY-MM-DD, MM/DD/YYYY, DD-MM-YYYY, YYYY/MM/DD, as well as ISO 8601 with time (YYYY-MM-DDTHH:MM:SS). The tool auto-detects the format from your input.

Is this Julian Date converter free?

Yes, completely free. No registration, login, or subscription required. All calculations run in your browser using JavaScript — no data is sent to any server, making it safe for sensitive date data.

What is the Julian Period?

The Julian Period is a chronological interval of 7980 years, beginning on January 1, 4713 BC. It was introduced by Joseph Scaliger in 1583 and is the basis for the Julian Day Number system used in astronomy. The current Julian Period will end in 3267 AD.

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Julian Date Converter
Calendar ↔ JDN Instantly

Convert calendar dates to Julian Day Numbers (JDN) and back — instantly and for free. Supports bulk input, file upload, Modified Julian Date, and one-click CSV download. No login needed.

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MJD

Modified Julian Date

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Bulk Julian Date Converter

Enter dates or Julian Day Numbers below (one per line) or upload a .txt/.csv file. Each line is converted independently.

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Today's JD (noon)

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Today's MJD

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Today (UTC)

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UTC Clock

Click to upload or drag & drop a .txt or .csv file containing dates

Supports .txt/.csv files up to 2MB

Date / Julian Day Number Input

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Output:

Calendar:

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Why Use Our Tool

Advanced Julian Date Converter Features

The most comprehensive free Julian Date tool — precise, fast, and packed with advanced conversion features for astronomers and developers.

Bulk Conversion

Convert dozens or hundreds of dates at once — one per line. Ideal for astronomical datasets, log processing, or batch date transformation tasks.

File Upload Support

Upload a .txt or .csv file containing your dates directly into the converter. Drag and drop or click to browse — supports files up to 2MB.

Auto-detect Mode

Automatically detects whether input is a calendar date or Julian Day Number. Supports YYYY-MM-DD, MM/DD/YYYY, ISO 8601, and JDN decimal formats.

Modified Julian Date (MJD)

Optionally display Modified Julian Date (MJD = JD − 2400000.5) alongside JDN — the compact format used in satellite tracking and space science.

CSV & TXT Download

Download conversion results as a structured CSV file or plain text. CSV format is ideal for importing into spreadsheets, databases, or analysis tools.

Real-time Validation

Input is validated live as you type. Invalid date formats, out-of-range values, and malformed Julian Numbers are flagged instantly before conversion.

Simple Process

How to Convert Julian Dates

Convert calendar dates to Julian Day Numbers (or back) in four simple steps — no software installation required.

Enter or Upload Dates

Paste calendar dates or Julian Day Numbers into the input box (one per line) or upload a .txt/.csv file. Mixed inputs are supported.

Choose Options

Select conversion mode (auto, Calendar→JD, or JD→Calendar), output format, calendar system, and whether to include MJD.

Click Convert

Hit "Convert Julian Date" and the tool instantly calculates each Julian Day Number or calendar date, displaying results with full detail.

Copy or Download

Copy results to clipboard or download as CSV or TXT. Use per-line breakdown to inspect individual conversions and identify any errors.

What Is a Julian Date? How It Works, Examples & Usage

A Julian Date (JD) is a continuous count of days elapsed since noon on January 1, 4713 BC in the proleptic Julian calendar — a system devised by scholar Joseph Scaliger in 1583. Unlike conventional calendar dates, Julian Day Numbers eliminate months, years, and leap-year complexity, making date arithmetic trivially simple: subtract two JDNs to find the days between them. Today (noon UTC) corresponds to JD 2460700+, and each passing day adds exactly 1.0 to this value.

How to convert a calendar date to Julian Date: Use the formula JDN = 367Y − INT(7×(Y + INT((M+9)/12))/4) + INT(275M/9) + D + 1721013.5, where Y is the year, M is the month, and D is the day. Our tool handles all edge cases — including the Gregorian calendar reform of 1582 — automatically. For a date like 2025-01-15, the Julian Day Number is approximately 2460691.

Who uses Julian dates? Astronomers rely on JDNs for ephemeris calculations and tracking celestial events across centuries. Satellite operators use Modified Julian Date (MJD = JD − 2400000.5) for compact storage. Software engineers use Julian dates for database date arithmetic. Historians use them to reconcile dates across Gregorian and Julian calendar eras.

Key variants: The Julian Day Number (JDN) is an integer — just the day count. The Julian Date (JD) adds a decimal fraction for time of day (0.5 = midnight, 0.0 = previous noon). The Modified Julian Date (MJD) shifts the epoch to November 17, 1858, providing a smaller, midnight-based number used in modern scientific applications. This tool converts between all three formats in bulk, instantly, and for free.

FAQ

Frequently Asked Questions

Everything you need to know about Julian Date conversion.

What is a Julian Date?

A Julian Date (JD) is a continuous count of days since noon on January 1, 4713 BC (proleptic Julian calendar). It provides a simple, calendar-independent way to calculate time intervals. Widely used in astronomy, navigation, satellite systems, and scientific computing.

What is the Julian Day Number (JDN) for today?

The live Julian Day Number for today is shown at the top of the converter tool above. As of 2025, today's JDN is approximately 2460690–2460720 depending on the exact date. Each calendar day increases the JDN by exactly 1.

What is Modified Julian Date (MJD)?

MJD = JD − 2400000.5. It shifts the epoch to November 17, 1858, 00:00 UT and uses midnight-based days instead of noon-based. MJD produces smaller numbers and is preferred in satellite tracking, GPS systems, and modern space science.

What date formats does this Julian converter support?

Supported calendar formats: YYYY-MM-DD, MM/DD/YYYY, DD-MM-YYYY, YYYY/MM/DD, and ISO 8601 with time (YYYY-MM-DDTHH:MM:SS or YYYY-MM-DD HH:MM:SS). Julian Day Number inputs (decimal numbers like 2451545.0) are also accepted and automatically detected.

Can I convert multiple dates at once?

Yes. Enter one date or JDN per line in the text area — all lines are converted simultaneously. You can also upload a .txt or .csv file for large-scale batch conversions. Results can be downloaded as CSV for further analysis.

Is this tool free and private?

Yes, completely free with no registration required. All calculations run entirely in your browser using JavaScript — no data is sent to any server. Your dates never leave your device, making this tool safe for sensitive or historical data.

What is the difference between Gregorian and Julian calendar output?

The Gregorian calendar (introduced in 1582) is the modern standard. The proleptic Julian calendar extends the Julian system backwards before its 46 BC introduction. For dates before October 15, 1582, the two systems diverge by several days. Our tool supports both and handles the 1582 reform boundary correctly.

What is an example Julian Date conversion?

January 1, 2000 at 12:00 TT (noon) = JD 2451545.0 (MJD 51544.5). January 1, 2025 = JD 2460676.5 (midnight) or 2460677.0 (noon). The Unix epoch (January 1, 1970, 00:00 UTC) = JD 2440587.5.

Julian Date ConverterCalendar ↔ JDN Instantly