Ensure that the data is sorted in date order.
Add a helper column (P, say) and in P2put =if(O2=O1 then 1,"") and copy
this down.
A duplicate date will then have a 1 against it.
You can then either use Autofilter to view those with a blank in P
(remember that the Autosum value inthe bar at the bottom of the screen
only counts visible cells); or you can re-sort by col P to keep the
"blanks" at the top and just work on those.

Alf