I have again been looking at this problem, and have come to the following conclusion / resolution:
No matter how i worked the custom formatting (or chose a pre-formated), Numbers kept switching months and day values (for day values below 13) and made no changes for dates with day values above 13.

The solution finally was to change language/region on another computer, open the document there, and make a format change so the numeric month values displayed as text and in a US format (IE , 12=Dec, Dec.14 2017) copying this into my file on my US region iMac, kept the correct formating intact.
My thanks to all who weighed in on this, and very much also to Wayne Contello Who suggested to Change the language/region.