Custom time formatting - PM/AM turning into P4/A4

[DashboardNovice]

I'm starting with the date/time in D4.

I'm trying to display PM/AM. In the customer format window, when I type PM/AM it goes in as Pm/\Am. After clicking the 'OK' button, it displays as P4/A4.

What is happening here?

 

[vletm]

DashboardNovice​

There are rules how do Excel works ...
You've there in Custom Formats few samples, how do those works.
It's AM/PM ... it's totally different than Your version.