date(1) should add "Julian day number" or the "Astronomical day
number" (see (info "(emacs) Calendar Systems") output format.
Else for crontab(5) one must do
# Run once per 48 hours (every other day), even across week, month, and
year boundaries:
33 22 * * * expr $(date +\%s) / 60 / 60 / 24 \% 2 > /dev/null &&
echo Check the toilet tissue.
Instead of just
33 22 * * * expr $(date +\%J) \% 2 > /dev/null && echo Check the
toilet tissue.
(assuming %J is the Julian day number.)
Currently you have day of the week, day of the month, day of the year,
seconds from second #1. Years from year #1, but no days from "day #1".
