days-between-dates Solver equation for 17b

[Massimo Gnerucci]

(10-08-2018 02:22 PM)Thomas Okken Wrote:  

(10-08-2018 11:01 AM)Massimo Gnerucci Wrote:  And maybe a day was added to augustus (taking it from february) to make it equal to iulius (uncertain).

Uncertain? I always thought that this was known for certain, but I'll admit I'm no expert and not familiar with any primary sources. At least it's plausible because it explains the break in the pattern of alternating 31- and 30-day months, if you assume that August / Sextilis used to be 30 days, September 31, October 30, November 31, and December 30. Making August 31 days, and changing September through December in order to restore the alternation as much as possible, meant that one day had to be stolen from somewhere, since three months were changed from 30 days to 31, while only two months were changed from 31 days to 30. So February, which was already the odd one out at 29 days, ended up with only 28. If this isn't rigorously documented, I guess that's because the calendar that was being modified, the Julian calendar, hadn't been in force for all that long, so it wasn't like they were messing with ancient tradition -- Caesar had taken care of that already.

Plausible it came from February, I concur. But it seems the only source is Johannes de Sacrobosco's Computus.