Require fromula to count instances of times and dates

Hi, I need a formula that counts individual instances of times and dates within a list and displays a count number next to them.

Example

Date Time Instance

08/08/14 10:00 1
08/08/14 10:00 2
08/08/14 10:00 3
08/08/14 10:00 4
08/08/14 11:00 1
08/08/14 11:00 2
08/08/14 12:00 1
08/08/14 12:00 2
08/08/14 12:00 3
08/08/14 13:00 1
08/08/14 13:00 2

Thanks in advance.

JD

Hi, your question is a bit ambiguous, but based on your example (with list in A2:A12), in B2:
=IF(A2=A1,B1+1,1)