It is clumsy creating a formula that knows where the last bit of data is for a date. It would be more convenient if you put your counts in the same row as the date. If so, you could use a formula like:
=IF($A1="","",COUNTA($C1:$C$20)-SUM(D2:D$20))
Put the above formula in cell D1 and copy it down. Change the reference to row 20 to somewhere beyond the end of your data.
Brad
