In a recent project, I made use of the "date -v ..." capability of FreBSD's date(1) command, which came in very handy to handle "yesterday's log files". I subsequently discovered "date -v" seems to be a FreeBSD-only feature and that GNU date (and therefore Linux) does not support it.
Considering how useful it seemed to me, I decided to try and port the functionality to GNU date -- changing the original FreeBSD code as little as possible.
This turned out to be far less of an issue than I anticipated. vary.c needed only a few small tweaks to build on Linux and within the coreutils framework. Hooking everything into date.c wasn't much harder. The result can be found here: https://github.com/mmayer/coreutils/tree/feature/vary
$ uname -sr
$ ./src/date --version | head -1
date (GNU coreutils) 8.21.131-cba81e-dirty