This package consists of a C library and a Perl module (which uses
the C library, internally) for all kinds of date calculations based
on the Gregorian calendar (the one used in all western countries today),
thereby complying with all relevant norms and standards: ISO/R 2015-1971,
DIN 1355 and, to some extent, ISO 8601 (where applicable).

WWW: https://metacpan.org/pod/Date::Calc