Julian dates

[FlexibleLearning.com]

Bob Sneidar wrote...

Not to throw a wrench in the works, but wasn't there some discussion in the
past about different versions of Julian dates? Which one is the more widely
accepted? I suppose the question could be formed, which one does Microsoft
use for Excel? :-)



Hi Bob

There is only one Julian Date system, but there are 2 methods of calculating
the corresponding date. Either method is good for dates after the switch to
the Gregorian calendar (1752 or 1582 with many exceptions). The difference
between the two depends on whether you want the proleptic-Gregorian (which
incorrectly assumes a continuation of the modern Gregorian calendar before
this year), or the Julian calendar method for early dates. Of course, you
also need the appropriate corresponding algorithm to convert the Julian Day
Number back to a date.

(Note that the 'Julian Calendar' and 'Julian Day Numbers' are not related in
any way... damn stupid naming system that has caused and continues to cause
no end of confusion.)

The 2 different methods explain why there are 2 different dates quoted for
Julian Day Number 0, as there are 2 different assumptions. 

The most common conversion algorithms are proleptic-Gregorian which is fine
unless you need historical accuracy... 14 October 1066 was actually a
Saturday, but most people quote a Tuesday because they use the 'wrong'
algorithm. They also end up with dates that would not have existed due to
the change in leap year calculations.

One final point... All conversion algorithms (to/from the proleptic/Julian
system) assume the first day of the year to be 1 January. Not the case. For
the English-speaking world, the year actually started on Lady Day 15 March
until 1752 (which became 6 April, hence the tax year date), and hence the
oft-used notation such as "1 Jan 1765/1766" to avoid confusion. Google "Lady
Day date" to see what I mean.

Finding a pair of 'true-Julian calendar' algorithms, however, is actually
quite hard and it took me a long time.

All the above is of academic interest if you work with 'modern' dates only.
Both ChartMaker and FieldFormatter, however, do the job properly and will
give the correct date and day of the week for both 'modern' and 'historical'
dates.

Excel uses proleptic-Gregorian, since you asked.

Hugh Senior
FLCo