Weekly Status Report from a monthly one

I have an external data source that tracks events opened and closed and keeps a running total for each month. I want to be able to do a weekly status of the events opened and closed. As the data source is a running total I need to find the difference per week i.e minus previous weeks from this weeks events. Can anyone thinkof a way to do this because it must roll on weekly. I then will link this to another page to display the info for that week. I thought i could have a live data feed and also an archieve sheet and work it out from this. Any ideas??