I've struggled with this issue for some months now. I have a workaround but it is not useable in every application.
In the attached list, imported into Excel (Office 365) from a CRM, the format cannot be changed and the dates cannot be used in any date related formulae.
Both the CRM and Excel have Locale set to English (United Kingdom).
I really need to understand the cause of this formatting issue in order to have the opportunity of an absolute solution.
My workaround is to apply 'Text to Columns' and set format to date.
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