A family of Microsoft word processing software products for creating web, email, and print documents.
Hmmm. To check the claim on the Microsoft site that you linked, I have changed the regional settings to English US (which changes the date format as you are undoubtedly aware) and changed the date formats of the content controls to US (easy to do with https://www.gmayor.com/insert_content_control_addin.htm - see below) and the additions from the example remain as 7 and 10 days.
When working with content controls. it is not so much the control panel regional date format that determines the veracity of the calculation, but the format applied in the date controls' properties. In the quoted example all the dates are displayed using Date content controls. You can format those controls in any combination of UK or US English date formats and the calculations always produce the correct results as formatted regardless of regional short date setting.