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You can use the attached as an example on how to do this.
I take the 'floor' of the value, which rounds down and just gets the whole number. The decimals here represent time which is not needed based on your post.
The excel number represents number of days since 1900. You can use the datetimeadd function to add that number to 1/1/1900. We need to subtract 2 here due to some nuances with leap years and how the date functionality works.