bug#80974: Add Time conversion specifiers quick reference

P=C3=A1draig Brady <[email protected]> writes:

>> And be sure to include the one liner script so people can do it
>> themselves. And be sure it's a real one liner.
>
> A portable one liner could be:
>
> printf $(printf '\\%03o\\n' $(seq 97 122) $(seq 65 90)) | xargs -If date =
+"%%f  %f"
>
> Though that's a bit obtuse.
> Note we had something similar with `dircolors --print-ls-colors`
> where we previously had a one liner to display the database.
> I'm considering instead we should support date --show-formats
> which would generate the above table with the current date/time/locale.
> It could also show the %E and %O variants (if different).
> It might also show the --iso and --rfc formats too.

My concern with a one liner or table of examples is that it risks being
misleading for, in my opinion, very little benefit.

An easy example is %H and %I which look the same for half of
day:

    $ for spec in H I; do date -d 12:00 +%$spec; done
    12
    12
    $ for spec in H I; do date -d 13:00 +%$spec; done
    13
    01

The --help output and info page explanations are fairly short and
adequate enough that one is unlikely to pick the incorrect specifier.

Collin