Without utilizing VBA, all you can really do is check to see if the cell contains a number.
=IF(ISNUMBER(D11),"a","")
Dates are simply numbers. 1 Jan 1900 = 1 and today = 44045. It's been that many days since 1 Jan 1900.
Format the cell with the formula as Marlett. if you want to keep what you had...
=IF(ISNUMBER($D11),"√"," ")
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