DateDelta

Julian Date Calculator

A single continuously increasing day count, sidestepping months, leap years, and calendar quirks entirely.

Year

Month (1–12)

Day

Julian Day Number

2,461,292

JDN counts consecutive days since noon UTC, January 1, 4713 BC.

What you entered

Year: 2026Month (1–12): 9Day: 8

1
Apply the Gregorian-to-Julian Day Number algorithm
f(2026, 9, 8)= 2,461,292

Result

Julian Day Number: 2,461,292

2026-9-8 corresponds to Julian Day Number 2461292 — used in astronomy to sidestep calendar irregularities like leap years and varying month lengths.

Julian Day Numbers for notable dates

The Julian Day Number counts days continuously from 1 January 4713 BC, which is why astronomers use it for date arithmetic.

Date	Julian Day Number	Note
1 January 1900	2415021	Start of the 20th century
20 July 1969	2440423	Apollo 11 Moon landing
1 January 2000	2451545	The J2000.0 epoch
1 January 2020	2458850	Recent reference
1 January 2026	2461042	Current era

The point of the system is that subtracting two Julian Day Numbers gives the exact number of days between the dates without any calendar arithmetic - no month lengths, no leap years, no Gregorian reform to work around. 2451545 for 1 January 2000 is the J2000.0 standard epoch that most modern astronomical coordinates are referenced to. Strictly the Julian Day begins at noon rather than midnight, so a precise value carries a .5 fraction for midnight dates, which is why some sources quote 2451544.5 for the same instant.

Why astronomers prefer it

Calculating the number of days between two calendar dates requires handling varying month lengths and leap years. Julian Day Numbers are just one continuously incrementing integer, making elapsed-time math between any two dates a simple subtraction.

Not the same as a 'Julian calendar' date

Despite the similar name, Julian Day Number isn't about the old Julian calendar (which the Gregorian calendar replaced) — it's a separate day-counting system, named after Joseph Scaliger's father Julius, still used today in astronomy and some scientific computing.

Frequently asked questions

What is the Julian Day Number for today?

Enter today's date above — as of mid-2026, it's approximately JD 2,461,580. Julian Day Numbers have been counting continuously since 4713 BC, making date arithmetic trivial: the days between any two events is just one subtraction.

How is a Julian Day Number different from a Unix timestamp?

A Unix timestamp counts seconds since January 1, 1970. A Julian Day Number counts days since January 1, 4713 BC. JDN is used in astronomy and historical dating; Unix timestamps are used in computing. They serve different communities but solve the same problem: making date math simple.

Why do astronomers use Julian Day Numbers instead of regular dates?

Calculating the days between two events across different calendar systems (Julian calendar, Gregorian calendar, different eras) is nightmarish with regular dates. JDN sidesteps all of it — just subtract the two JDN values to get elapsed days. It's been the standard in astronomy since 1583.