Formula comparing cell to fixed date always returns false
Solution 1:
Take the quotes off of the dates in your formula. It is trying to find the greater of two text values instead of the greater of two date values.
Solution 2:
if your S8 is date,
try this
=IF(S8 = Date(yyyy;mm;dd);"Yes";"No")
if your S8 is date, compare it with date as well...