Inverse JD (Julian Date) and JD Number Calculator
PHP Program by Jay Tanner

Enter any positive JD Number or Julian Date in the range from
JD = 0.0   to   JD = 5373484.5    to compute the corresponding
dates and times on the Julian and Gregorian calendars

Calendar Input Mode:    Julian              Gregorian 
J   2024     February
      1 2 3 4
5 6 7 8 9 10 11
12 13 14 15 16 17 18
19 20 21 22 23 24 25
26 27 28 29      
G   2024     February
        1 2 3
4 5 6 7 8 9 10
11 12 13 14 15 16 17
18 19 20 21 22 23 24
25 26 27 28 29    

For JD (Julian Date) 2460363 on the Gregorian Calendar, the
corresponding JD number, calendar date and time is:

JD (Julian Date)JD NumberGregorian Calendar DateTime

The Julian Day Number and the Julian Date are NOT the same thing nor is the JD number simply the integer part of the Julian Date value.

Technically, the JD number equates to the Julian Date at 12:00 noon on the date.  This is the only time of day when the Julian Date and the JD number happen to be numerically equivalent.

Given any general Julian Date (JD) the corresponding Julian Day number (JDNum) is:
JDNum = floor(JD + 0.5)

The JD number (Julian Day Number) is ALWAYS an integer value. It is simply the sequential serial count of the number of calendar days since the mathematical origin of the respective calendar system to which it applies. It acts as a unique sequential serial number for each date on the calendar system.  However, the only thing that the JD number tells us is the date and the day of the week on the calendar. For some calendrical computations, this is sufficient.

When the time of day needs to be included in a calendrical computation, then we use the general Julian Date, usually simply abbreviated as JD, with the fractional part of it indicating the time of day on the given date.

The Julian Date (JD) contains information for the calendar date, day of the week and the time of day.  It is usually a fractional value and only equates to an integer value at noon on the corresponding date, at which point it also equates to the standard JD Number for that calendar date. 

Calendar        Mathematical Origin (JD = 0.0,  JDNum = 0)
---------  ----------------------------------------------------
Julian     −4712 Jan 01   =  4713 BC Jan 01 Mon 12:00:00.000 TT
Gregorian  −4713 Nov 24   =  4714 BC Nov 24 Mon 12:00:00.000 TT
Given any general JD (Julian Date) value, the corresponding JD number value and day of the week (DoW) index (from 0 to 6) can be found by:
JDNum = floor(JD + 0.5)

DoW = (JDNum + 1) mod 7

The DoW indices are: 0=Sun, 1=Mon, 2=Tue. ..., 6=Sat
The DoW index formula applies equally to both the Julian and Gregorian calendars.

Jay Tanner - 2024