Date and Time on the Top Menu

Do you mean as a menu item in your navigation?
I would recommend adding it to your header – depending on what format you want to out the date and time in.

Add this to your template:
<?php echo “Welcome, it is ” . date(‘d-m-y h:i:s’); ?>

But you can choose how to output:
https://www.php.net/manual/en/function.date.php

I’m using a child theme. Where should I put that code? I’d like the time and date to vary depending on the visitors’ location, not mine.

Add it to your header.php under:
Appearance –> Themes –> select your child theme –> Editor –> select header.php

If you want to use the localtime of the browser, then you could use JavaScript like this:
https://isabelcastillo.com/display-current-date-javascript
or
https://www.wpbeginner.com/wp-tutorials/how-to-display-todays-date-in-wordpress/#comment-310613

Just create a div that matches the ID name and output in the document.getElementById.
In the example above, they use currentDate.

Thank you very much!