bug(date): Mixing flags part 2, accepted

Hi, uutils mainteners

one thing i open a new issue instead staying with -s and -d one and updating the title is that one is a bad bug, these one not, so will be in part 2 whiches harmless

example 1 test

relunsec@relunsec:~/software/coreutils/target/debug$ ./date --rfc-3339 ns -R -d ""
Fri, 18 Sep 2026 00:00:00 -0400
relunsec@relunsec:~/software/coreutils/target/debug$ gnudate --rfc-3339 ns -R -d ""
gnudate: multiple output formats specified
relunsec@relunsec:~/software/coreutils/target/debug$ 

example test 2

relunsec@relunsec:~/software/coreutils/target/debug$ gnudate -r . -I +hours
gnudate: multiple output formats specified
relunsec@relunsec:~/software/coreutils/target/debug$ gnudate -r . -I hours
gnudate: the argument ‘hours’ lacks a leading '+';
when using an option to specify date(s), any non-option
argument must be a format string beginning with '+'
Try 'gnudate --help' for more information.
relunsec@relunsec:~/software/coreutils/target/debug$ ./date -r . -I hours
2026-09-18T07-04:00
relunsec@relunsec:~/software/coreutils/target/debug$ 

example test 3

relunsec@relunsec:~/software/coreutils/target/debug$ ./date -R -R -R
Fri, 18 Sep 2026 09:33:41 -0400
relunsec@relunsec:~/software/coreutils/target/debug$ ./date -R -R -R -R
Fri, 18 Sep 2026 09:33:43 -0400
relunsec@relunsec:~/software/coreutils/target/debug$ ./date -R -R -R -R -R
Fri, 18 Sep 2026 09:33:46 -0400
relunsec@relunsec:~/software/coreutils/target/debug$ gnudate -R -R -R -R -R
gnudate: multiple output formats specified
relunsec@relunsec:~/software/coreutils/target/debug$ 

Start at the date command's option parsing and output-format validation, using the three command-line examples and GNU date as the behavioral reference. Add regression coverage for conflicting formats, invalid format arguments, and repeated -R flags; done means the Rust date command reports the same conflicts and validation errors as gnudate.