There is a distinction to be made between the JD (Julian Date) and the JD
(Julian Day) Number. They are not always numerically equivalent but can
sometimes be confused as such because they can sometimes be equivalent.
The JD (Julian Date) corresponds to a specific date and 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. This is the only time that the Julian Date
and the JD number happen to be numerically equivalent.
The JD (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.
Every date on either the Julian or Gregorian calendar system has a unique
sequential date serial number called the Julian Day Number
, always a positive integer, usually simply called the JD number
, to distinguish it from all other dates on the calendar. The only thing the JD number tells us is the date on the calendar. When the time of day is involved, then we use the general Julian Date
, usually abbreviated to JD
, with the fractional part of it referring to the time of day on the given date.
Calendar Mathematical Origin (JD = 0.0, JD Num = 0)
Julian -4712 Jan 01.5 = 4713 BC Jan 01 Mon 12:00:00.000
Gregorian -4713 Nov 24.5 = 4714 BC Nov 24 Mon 12:00:00.000
Given any general JD (Julian Date) value, the corresponding JD Number
value and day of the week (DoW) index are found by:
JDNum = floor(JD + 0.5)
DoW = (JDNum + 1) mod 7
The DoW indices are: 0=Sun, 1=Mon, ..., 6=Sat
The DoW index formula applies equally to both the Julian and Gregorian calendars.