Update: Time::Local 1.30 includes a new pair of functions timegm_posix/timelocal_posix which address all issues outlined in this article, including the issues with the traditional functions, at least as pertains to Time::Local’s purpose as an inverse of the gmtime/localtime Perl builtins.

The new _modern function variants in Time::Local have come up a few times lately. I have some thoughts on them, but presenting my position dispassionately enough to be persuasive demands an essay of unfortunate length… so let’s get on with it.

Let me lead with the positive: it is a problem with the traditional functions that they would sometimes add 1900 to the year and sometimes a different value and sometimes nothing. This heuristic in the interface is bad. Doing something about it is a good idea.

Let me also lay out some simple statements of fact: Perl ships with gmtime and localtime functions which return a datetime represented as a list of numbers. Time::Local supplies inverse functions which take such a list and return the Unix epoch time that corresponds to that datetime. The Perl functions, among other things, return the year as the number of years since 1900. A correct inverse of the core functions would therefore simply add 1900 to the year passed.

The traditional Time::Local functions do not fully do this: they only do it if the year number is just big enough but not too big.