You recently said:
> The only thing I can think of is to choose some large negative date
> (4004 BC?), and count everything from there.
I'd also suggest the Julian date scheme which is days from its rather old
start date (chosen to be before the start of history). It does avoid issues
around which calender you are using (Julian / Gregorian) and when the year
starts (Jan 1st, Mar 25th, Good Friday, Easter Day, Christmas Day etc). Equally
people can convert it to the calendar they prefer to use.
That bit about starting at midday is a more recent convention for use by
astronomers. I think the original started at midnight. (Otherwise you have
the problem that the day in the morning is different from that in the
afternoon. Astonomers have the opposite problem in that they don't want the
day to change at midnight.) Year 0 is a recent fudge invented to help
calculations - beware of conversion routines that do or don't include it,
otherwise your dates could be out.
One possible problem is vague and uncertain dates. If you only know the date
to a certain month, the exact day number gives a spurious sense of accuracy.
Tim Partridge. Any opinions expressed are mine only and not those of my employer