After looking into how fprintftime affects GNU tar, I decided to change
GNU tar to not use fprintftime.
The initial impetus for this new change was that recent changes to
Gnulib broke the GNU tar build on macOS, and fixing that in the obvious
way would have caused fprintftime to drag in some extra multithreading
code that is not needed (tar is single-threaded). There didn't seem to
be a convenient way to prevent that.
But also, if GNU tar is ever to count columns of output (something Bruno
mentioned), it can't use fprintftime because fprintftime returns a byte
count, not a column count.
So I changed GNU tar to simply call the system strftime. (Calling
nstrftime, or even nstrftime-limited, seems like it would also drag in
the unwanted multithreaded code.) This is good enough for GNU tar's
purpose. At some later point, someone can take up the issue of counting
columns vs counting bytes.
https://cgit.git.savannah.gnu.org/cgit/tar.git/commit/?id=453d903de916a9f705398152cec9db369b7d7bb8
