SE,
let us assume that you want to count the number of John's on
10-11-2005. Consider that a separate cell contains the Person (e.g. F1
contains John) and another separate cell contains the date (e.g. G1
contains 10-11-2005). Now, if we want to put this formula in H1,

H1: =SUM(IF($A$1:$A$20=G1, 1, 0)*IF($C$1:$C$20=F1, 1, 0))

This is an *array* formula. To work you must use Shift+Ctrl+Enter. If
you do not do the key combination correctly, select the formula cell,
click on the formula bar and repeat the key combination. When finished
you will see brackets around the formula in the formula bar.

HTH
Kostis Vezerides