assume your data range is from A1:A100
In B1 type =min(a1:a100) this will tell you the earliest date.
In B2 type =max(A1:A100) tells you latest date.
Now from B3 downward until you've covered the min and max dates, type (or
autofill or by formula) a date for every month that needs to be counted.
Starting in C3,
=SUMPRODUCT((A1:A100>=DATE(YEAR(B3),MONTH(B3),1))*(A1:A100<=DATE(YEAR(B3),MONTH(B2)+1,0)))
copy/paste this formula down next to each date in B3 the bottom of your list.
And, if you need a formula to fill down the list
in B3 =min(A1:A100)
in B4 =date(year(b3),month(b3)+1,day(1)) copy/paste this down
must give credit to
http://www.beyondtechnology.com/tips100.shtml for the
way to find beginning of and end of month dates w/o using eomonth()