This answer and comments mention --rfc-3339 and a hidden --iso-8601 option that I have used for a long time and now seems to be undocumented.When did that option documentation get removed from the --help text?Will the option go away anytime soon?
date command --iso-8601 option
date
The option was introduced in the coreutils date (which is probably what you have) in 1999 (Apr. 8).The documentation was removed in 2005 without much explanation in the commit.In 2011, the help for --iso-8601 was reintroduced with the following explanation:We deprecated and undocumented the --iso-8601 (-I) option mostlybecause date could not parse that particular format.  Now thatit can, it's time to restore the documentation.* src/date.c (usage): Document it.* doc/coreutils.texi (Options for date): Reinstate documentation.Reported by Hubert Depesz Lubaczewski in http://bugs.gnu.org/7444.It looks like the help was taken out in version 5.90 and put back in, in version 8.15 (it is not in my 8.13) and the comment above suggests that it is now back to stay and not likely to be disappearing any time soon.