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