date: Fix Error message missing '+' for format string after valid d flag

[cerdelen]

After passing a valid date with --date a user can provide a Format string. If this string is not starting with a '+' a specific error message is printed.

Fix: #10920

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/tail/inotify-dir-recreate (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/misc/io-errors is no longer failing!
Note: The gnu test tests/pr/bounded-memory is now being skipped but was previously passing.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/pr/bounded-memory (fails in this run but passes in the 'main' branch)
Note: The gnu test tests/cp/link-heap is now being skipped but was previously passing.
Note: The gnu test tests/unexpand/bounded-memory is now being skipped but was previously passing.

[cakebaker]

Thanks!