is the 1st Jan 1, or other convenient dates like the 1st Jan 1970 or
-The dateutils reference day (at the moment) is the 31st Dec 1916, or
-equivalently the Naught-th of Jan 1917. This may look like an
+The (internal) dateutils reference day is the 31st Dec 1600, or
+equivalently the Naught-th of Jan 1601. This may look like an
arbitrary date but it has some interesting properties:
- it is a Sunday, so weekday computation is easy as it is a mod 7
-- the year is 1 mod 4, so the number of leap years between 1917 and
- another year Y is simply Y div 4.
-Actually any date that suffices these properties is fine but 1917 is
-the first after 1900, and dates <= 1900 that have different leap year
-rules so cannot be as easily used as our reference date.
+- the year is 1 mod 4, so the number of leap years between the reference
+ year and any other year Y is simply Y div 4.
+In earlier versions we used 1917 for the reference year because it's the
+first year after 1900 that meets the conditions above. But then we
+relaxed leap year rules to support dates beyond 2100 (2100 isn't a leap
+year) so for symmetry reasons we should support years below 1917 in a
The daisy calendar has no notion of years or months.