I installed the package macros/latex/contrib/supported/datenumber by Jörg-Michael Schröder.
From the documentation:
This package provides commands to convert a date into a number and vice versa. Additionally there are commands for incrementing and decrementing a date. Leap years and the Gregorian calendar reform are considered.
The license is LPPL.
Reinhard Zierke for the CTAN team