Looking at day of the week, via SRFI-19's date-week-day: scheme@(guile-user)> (use-modules (srfi srfi-19)) scheme@(guile-user)> (julian-day->date 1721426 0) $1 = #<date nanosecond: 0 second: 0 minute: 0 hour: 12 day: 1 month: 1 year: 1 zone-offset: 0> scheme@(guile-user)> (date-week-day (julian-day->date 1721426 0)) $2 = 1 scheme@(guile-user)> (date-week-day (julian-day->date 1721425 0)) $3 = 6
The output for 0001-01-01, Monday, is correct. The preceding day is actually a Sunday, but Saturday was shown. Looking at the code, this bug arises for the same reason as the problem with date-year-day raised in bug#26151. The date-year value, of the weird zero-skipping year numbering, is passed to an algorithm that obviously expects astronomical year numbering. Looking at the code also reveals a second problem: the algorithm is written to perform divisions with quotient where it obviously needs modulo. This will manifest in erroneous computations for some earlier years once the above is fixed. -zefram