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From: James Kuyper <jameskuyper@alumni.caltech.edu>
Newsgroups: comp.lang.c
Subject: Re: "A diagram of C23 basic types"
Date: Mon, 28 Apr 2025 23:34:53 -0400
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On 4/28/25 20:10, Janis Papanagnou wrote:
> [ Just noticed this post while catching up in my backlog, so I'm not
> sure my questions/comments have already been addressed elsewhere. ]
>
> On 16.04.2025 22:04, Scott Lurndal wrote:
>>> [...]
>>
>> Back in the mainframe days, it was common to use julian dates
>> as they were both concise (5 BCD digits/20 bits) and sortable.
>>
>> YYDDD
>>
>> If time was neeeded, it was seconds since midnight in a reference
>> timezone.
>
> I don't quite understand the rationale behind all that said above.
>
> "YYDDD" was used without century information? How is that useful?
> (I assume it's just the popular laziness that later lead to all the
> Y2k chaos activities.)
>
> And "seconds since midnight" where taken despite the Julian Dates
> have a day start at high noon (12:00)? [*]

Strictly speaking, "Julian Day" is the number of days since Jan 01 4713
BCE at Noon (a date that was chosen because it simplifies conversion
between several different ancient calendar systems. It starts at Noon
because it was devised for use by astronomers, who are generally awake
at midnight and asleep at Noon (especially in ancient times).

Informally speaking, "Julian Day" is commonly used to refer to any
system for designating dates that include a "day of year" component, as
does the above example. Most of these start at midnight, not Noon.

There's no use being a purist about this (that would be my preference
too) - the informal meaning is quite common, probably more common than
the "correct" one.