Excel handles Date values as integer numbers. Number one means 1st of January 1900, two means 2nd of January 1900 and so on. Today (27th of February 2013) is day number 41332. So date number can be understood as the number of days since 31st of December 1899.
The first day that Excel supports is 1st of January 1900 and if you need earlier dates, Excel’s build in date functions are not working.
Excel actually supports two different Date Systems: In addition two this 1900 Date System you can also use 1904 Date System where number one means 1st of January 1904. This can be selected on Calculation settings but I recommend to use 1900 Date System to maintain better compatibility to e.g. other spreadsheet programs.
Excel handles times as decimal numbers between 0 and 1 and so 41332,41667 means that the date is 27th of February 2013 and time of the day is 10AM.
Following video demonstrates this.