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

[David Jones]

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?