I saw this occasionally with FMP 12.
Tested on Windows7 and not occurred.
I found that shortcut Ctrl+- for insert current date doesn't work with minus in ten key,
but same key work for zoom out in interactive container.
"I have an interactive PDF container on various layouts. I just discovered that if such an object is on a layout, I can no longer use the Command-Minus keyboard shortcut to insert today's date."
Do you actually mean that the presence of such an object on a layout changes the behaviour of the keyboard shortcut? If so, that seems a bit bizarre.
Yes. This doesn't happen unless there is an interactive container on the layout. Otherwise, the command functions normally.