A very simple way to accomplish this is to put in a "helper column"
consisting of:
= text( year( your_date_cell), "####") & " - " & text( month(
your_date_cell), "##")

Then just let Excel subtotal (Data / Subtotal) "Count" on changes in
the helper column. This assumes that the entries are arranged
chronologically in the column containing "your_date_cell".

Chris