According to the docs for strftime the %P modifier isn't supported on macOS. From some testing it seems that %p is supported but incorrectly output the result as uppercase instead of lower.

This means that uses of the %P or %p modifier in format strings passed to the userdate function won't output the expected strings on macOS.

We can add a similar fix to calendar/type/gregorian/classes/structure.php in the timestamp_to_date_string function which is already correcting the %I modifier.