**NOTE:**
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.