Julian Date Converter: Gregorian to JD Calculator

Julian Date Converter

Convert standard Gregorian calendar dates into Julian Dates (JD) used by astronomers for precise time calculations.

Select Gregorian Date

Julian Date (JD) --

What is a Julian Date?

The Julian Day is a continuous count of days since the beginning of the Julian Period. It is used primarily by astronomers and in software to easily calculate the time elapsed between two events.

Why Do Astronomers Use It?

Standard calendars are complex due to leap years, varying month lengths, and historical calendar shifts (like the transition from Julian to Gregorian). Julian Dates simplify this by providing a single, monotonically increasing number.

Frequently Asked Questions (FAQ)

When did the Julian Day start?

The Julian Day number 0 was assigned to the day starting at noon on January 1, 4713 BC in the proleptic Julian calendar.

What is a Modified Julian Date (MJD)?

MJD is JD - 2400000.5. It is used to keep the numbers smaller and to make the day start at midnight instead of noon.

How it Works

The converter applies the standard integer Julian Day Number algorithm. With a = floor((14 - month) / 12), y = year + 4800 - a, and m = month + 12a - 3, the Julian Date is: JD = day + floor((153 × m + 2) / 5) + 365 × y + floor(y / 4) - floor(y / 100) + floor(y / 400) - 32045. The terms floor(y / 4) - floor(y / 100) + floor(y / 400) encode the Gregorian leap-year rules, while the constant 32045 anchors the count to the Julian Period epoch. Example: January 1, 2000 converts to JD 2451545.