How to have GNU's date in OS X?
The gdate binary from coreutils should be the gnu date program:
$ gdate --version
date (GNU coreutils) 8.25
Copyright (C) 2016 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
Written by David MacKenzie.
If you don't have that in the path, perhaps brew rm coreutils && brew install coreutils would be a place to start as well as inspecting your $PATH