number of open actvities each end of month - closing balance, running totals

Hello,

I'm using sql 2008 R2 standard edition.

I have a table containing activities with columns ActivityID, StartDate, EndDate. Few sample data:

acvtivityID          StartDate          EndDate

12345                 12-22-2013       01-04-2014 

12346                 12-28-2013       NULL

12347                 01-16-2014       02-17-2014

12348                 01-17-2014       03-28-2014

12349                 03-21-2014       NULL

Now i want a query which gives the count of open activities at each end of month, e.g.:

date          Openactivities

12-31-2013          21

01-31-2014          20

02-28-2014          12

03-31-2014          23

...etc..

so for example, on march 31, 2014 the number of open activities is the count(activityID) where startdate <= 3-31-2014 and the EndDate >= 3-31-2014 or EndDate IS NULL. And some way it has to be grouped by eahc end of month..

How to do this?

Regards, Hennie