Help needed.. How to calculate a range of cells after a specified date.

Im hoping someone can help.

I am looking to find the sum of all sickness within the last 12 rolling months.

I want to add sum instances of sickness after a certain date. I have placed an example of my data set below.

Dates
01/01/2011 S
02/01/2011
03/01/2011
04/01/2011 S
05/01/2011


So if I was searching all sickness after 02/01/2011 i would need the result to be 1.

Can anyone help?

Matt