Thanks for all the info, and some good advice too.
I can confirm that Minneapolis time is the time that NWA use for such matters. my Elite status changed exactly at midnight their time. I did phone the airline to enquire about and received that info PLUS something pretty handy for other people.
You don't need to phone to have the Status change recognised on the booking. Simply entering a "new" Worldperks number on your reservation via NWA site ( where "new" = the same number re-entered ) does the same job. That's the procedure that I carried out on all my future itineraries.
it worked too.

I'm a happy Plat too, upraded at boarding to exit-row seat upstairs .. mmmmmm sweet.