I'm the author of the Perl DateTime library. It works correctly as _documented_. Here's the first two paragraphs of the docs:

> DateTime is a class for the representation of date/time combinations, and is part of the Perl DateTime project.

> It represents the Gregorian calendar, extended backwards in time before its creation (in 1582). This is sometimes known as the "proleptic Gregorian calendar". In this calendar, the first day of the calendar (the epoch), is the first day of year 1, which corresponds to the date which was (incorrectly) believed to be the birth of Jesus Christ

Always RTFM.

Edit: I double-checked, and while this was not in the docs in the first release, it _was_ in the docs as of 2015.

https://metacpan.org/pod/DateTime