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),"√"," ")