Fix date for Ubuntu >= 25.10

[drwetter]

works for

• ubuntu 24.04
• Debian 13
• openbsd 6.6 / 7.x
• macos 15.7.1

Fixes #2909

What is your pull request about?

• Bug fix
• Improvement
• New feature (adds functionality)
• Breaking change (bug fix, feature or improvement that would cause existing functionality to not work as expected)
• Typo fix
• Documentation update
• Update of other files

If it's a code change please check the boxes which are applicable

• For the main program: My edits contain no tabs, indentation is five spaces and any line endings do not contain any blank chars
• I've read CONTRIBUTING.md and Coding_Convention.md
• I have tested this fix or improvement against >=2 hosts and I couldn't spot a problem
• I have tested this new feature against >=2 hosts which show this feature and >=2 host which does not (in order to avoid side effects) . I couldn't spot a problem
• For the new feature I have made corresponding changes to the documentation and / or to help()
• If it's a bigger change: I added myself to CREDITS.md (alphabetical order) and the change to CHANGELOG.md

[teki69]

@drwetter , it doesn't work for Ubuntu 25.10. Both date -r 735275209 and date -r @735275209 return current date without an error. So the first conditional statement still succeeds and sets HAS_OPENBSDDATE=true