We’re considering how this could be implemented across the group identity.Ben Fry commented
This is a show stopper and will mean moving our business off Office 365. I have spent 60-80 hours on a shared spreedsheet with dashboards and multiple date field to be continually broken by US regional date settings.
I give up and will move the business to Google Apps now.
Unbelievable that such a critical feature does not work in Office online.