In Camelot 3.x, on the main menu screen, there are Vital Stats numbers that give you a summary of how your company is doing.
One number that is often confusing is the "Last Month Sales to Date". The purpose of this number is so you can compare how you are doing this month with how you were doing the same time last month. For example, if the current date is October 5th, 2009, then the "Last Month Sales to Date" vital stat would be giving you the sales for September 1st through September 5th, so you could compare that to your current monthly sales to date, which would be Oct 1st thru 5th. This is really more useful than comparing to the entire previous month's sales.
There are lots of options for what you can show in the Vital Stats, including an entire previous monthly sales number, and one way to see all that is available is to right-click on the Vital Stats and choose Show All. You can reset to the program default with a right-click option there as well.
vital stats last month sales to date previous month sales sales to date main menu stats