Those familiar with the notion of
Julian Day (JD) – Continuous count of days since the beginning of the Julian Period, on 1 Jan. 4713 BCE in the proleptic Gregorian calendar
– ( J - JAD to JYT | WhatIs.com - Search Results | {1} ) –
will note that today’s JD is 2458888
and that that’s the number of days that have elapsed since January 1, 4713 B.C.
That system of date-keeping is useful in things like Astronomy for calculating “number of days” between two specific days.
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An interesting case in point relates to the birth and death dates for the actor James Dean:
Born Feb/8/1931 Sunday JD 2426381
Died Sep/30/1955 Friday JD 2435381
A difference of 9000 days (or 24.64 years)!
If he were still alive he’d be 32507 days old today (89 years old)
It can be fun/educational to look up other specific dates and see the “days elapsed” between them.
Who says it dates an event? I always thought the idea was to set the origin back far enough so that all historical observations would have a positive day number.
One Julian period is supposed to be 28 x 19 x 15 years, and you supposedly get 4713 BC if you go back to when all three cycles were last synchronised at 1, though now that you mention it I ought to double-check that calculation (Scalinger’s calculation is described in the Wiki article assuming that 1 bc was 9 in the solar cycle, 1 of the lunar cycle, and 3 in the 15-year indiction cycle.)
It’s difficult to find ONE (as opposed to several) website that goes into all the trivia about Julian Day (or Date as some label it) and the confusion generated by some of them makes a “simple answer” hard to find.
But from the reading I’ve been able to locate online, I would side with DPRK for the “why” of it all.
I just know that astronomers and others who deal with long stretches (multiple years) of details find it easier to deal with the Julian Day approach than with all the math to cope with BC and AD and various date-counting schemes in the past.