This is due to the fact the Dates are also stored as numeric notations in MS Excel formats.
You cannot directly apply formula in Smart Markers. Smart Markers have their own rules and grammar. As an easy workaround, I guess, once you have processed the Smart markers and data is filled into the relevant cells of the column, then you may set your desired Excel formula to the values manually using Cell.Formula attribute to get your desired strings. Here you may use Shared formula feature to apply the formulas in one go, see the document for your reference.