Time Tools

Julian Date Calculator

Convert between Julian date and Gregorian calendar. Calculate Julian day number for astronomy and date conversions.

Use Julian Date Calculator to get instant results without uploads or sign-ups. Everything runs securely in your browser for fast, reliable output.

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About this tool

Julian dates (Julian Day Numbers) count days since January 1, 4713 BC. Used in astronomy, history, and scientific date calculations for continuous day counting without calendar complexities.

Different from Julian calendar (old calendar replaced by Gregorian). Julian Day Number (JDN) simplifies date arithmetic - no months, years, or leap years to worry about.

Usage examples

Today's Julian Date

Current Julian day number

January 1, 2024 = JD 2460310

Historical Date

Convert historical events

July 4, 1776 = JD 2369916

How to use

  1. Enter a "Gregorian Date" (MM/DD/YYYY).
  2. Click "Calculate" to get Julian day number.
  3. Or enter "Julian Day Number" to convert back.
  4. Use for astronomy, history, or date math.

Benefits

  • Convert to Julian day number
  • Simplify date calculations
  • Astronomy compatibility
  • Historical date conversion
  • Continuous day counting
  • No calendar complexities

FAQs

What is a Julian date?

Julian Date (Julian Day Number) counts days since noon UTC on January 1, 4713 BC. Used in astronomy to avoid calendar complications. JD 0 = January 1, 4713 BC. JD 2,400,000 = Nov 16, 1858. Current JD is over 2,460,000. Simplifies calculations across calendar changes.

What is the difference between Julian calendar and Julian date?

Julian calendar: old calendar used before Gregorian (introduced by Julius Caesar, 45 BC). Julian date/day number: continuous day count for scientific use. Confusingly different! Julian calendar had leap year every 4 years (no century exception). Replaced by Gregorian in 1582.

Why do astronomers use Julian dates?

Benefits for astronomy: no time zones, no calendar changes, continuous numbering, simplified math (just subtract for days between events), works across centuries, and coordinates observations globally. Calculating "days between" is simple subtraction, no month/year complications.

How do I calculate days between two dates?

Using Julian dates: Convert both dates to JD, subtract. Example: Dec 31, 2024 (JD 2460674) - Jan 1, 2024 (JD 2460310) = 364 days. Much simpler than calendar math with varying month lengths and leap years!

What is Modified Julian Date (MJD)?

Modified Julian Date = JD - 2,400,000.5. Starts November 17, 1858, at midnight. Used for modern dates to avoid large numbers. MJD 0 = JD 2,400,000.5. Easier to write and remember. Space agencies and meteorology commonly use MJD.

When did the Julian calendar change to Gregorian?

Gregorian calendar adopted October 1582 (Catholic countries). October 4, 1582 (Julian) was followed by October 15, 1582 (Gregorian) - 10 days skipped! Protestant countries changed later: Britain 1752, Russia 1918, Greece 1923. This is why historical dates before 1582 can be confusing.

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