Emacs: Find number of days between dates

from blog lmno.lol @alvaro, | ↗ original
Needed to find the number of days between two dates. Emacs calendar must know this… Fire up the manual (M-x info-emacs-manual or C-h r). Info-goto-node (or g). Type "counting days" and voilá: To determine the number of days in a range, set the mark on one date using `C-', move point to another date, and type `M-=' (`calendar-count-days-region')....