Yesterday, Nov 17, was the sesquicentenary of the zero-date of the American Ephemeris. I meant to write, but got distracted. Astronomical ephemeris counts forward from this date.
That particular date was picked because it was (approximately) Julian Day 1,000,000, but given calendar shifts and all, one could argue for other zero dates as well. The important thing is to pick one.
There are some who think that this would be a better date to use as a zero-time computer timekeeping than what most of us use presently. It has the advantages that almost all of the Julian/Gregorian calendar skew comes after this (Russia being the major exception) and far enough back that nearly all time-and-date calculations you need to do with quick math can therefore be just adding and subtracting. And it has a nice scientific tie-in.
Other common zero-dates are 1 Jan 1904 (picked because if you pick this date, you can calculate all the way to 2100 assuming that leap years are every four years), and 1 Jan 1970 (picked because this was the last day that The Beatles recorded music in Abbey Road studios — actually, their last date was Jan 4, but close enough).