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".