[Info-vax] Why is starting epoch 17 Nov 1858?

[Stephen Hoffman]

On 2021-03-10 14:21:17 +0000, David Jones said:

> On Tuesday, March 9, 2021 at 3:02:13 PM UTC-5, Jim DeCamp wrote:
>> On Monday, November 27, 1995 at 3:00:00 AM UTC-5, 
>> zrep... at cuppa.curtin.edu.au wrote:
>> It's called Smithsonian time, and yes it is the epoch for the Modified 
>> Julian Day, which is the Julian Day minus 2,400,000.5 days.
> 
> Is that 2400000.5 magic number ensconced in a header file symbol 
> definition anywhere?

It's been a while since I've looked at that code, but I wouldn't expect 
to see that value around other than in comments, as it is immaterial to 
the process of converting to and from the base date.

Converting from 0 is 17-Nov-1858, and from 17-Nov-1858 produces 0. A 
day's accumulated centiseconds gets you 18-Nov-1858, obviously.

That there's a second and earlier date with a nice round offset from 
the any base date isn't any more relevant to the conversion process 
than would be a nice round offset date after the base date.

Neither the Smithsonian base date nor the Gregorian base date is 
anything other than a convenience, particularly given we don't know the 
offset from the Big Bang (yet?), and usually wouldn't want to store it 
everywhere anyway.

(Discussions of where and of when and of speed and of differing 
calendars all inherently involved when converting times and dates are 
all hereby ignored, for the purposes of this discussion.)


-- 
Pure Personal Opinion | HoffmanLabs LLC