Dates are stored in Excel as numbers known as date serials. This enables you to perform basic arithmetic such as calculating tomorrow's date simply by finding today's date and adding 1. If dates were stored as text, it would not be at all obvious that the 1st February is preceded by the 31st January. Storing dates as numbers also means they can be re-formatted.
The first date in Excel is the 1st January 1900, and has the date serial 1. All subsequent dates follow as consecutive integers, so the 2nd January 1900 has the date serial 2, and so on and so forth. This can be somewhat confusing as the serial for the 1st January 2012 is 40,909.
If you have been trying to manipulate dates in Excel and have started seeing numbers over 40,000 then you are seeing date serials. The way to resolve this issue is to right click on the offending cell and re-format it as a date.