Needed to find the number of days between two dates. Emacs calendar must know this…
To determine the number of days in a range, set the mark on one date using `C-<SPC>', move point to another date, and type `M-=' (`calendar-count-days-region'). The numbers of days shown is [inclusive]{.underline}; that is, it includes the days specified by mark and point.
Note: you can use the mouse to jump to another date, or "g d" (calendar-goto-date).