Free Online Microsoft Excel 2010 Training [From Microsoft]
To help you learn more about dates and formulas click the link below:
Have you ever wanted to subtract one date from another? Or find out how many workdays until vacation? Or maybe you need to figure out a project end date. You can do all of this and more by using date functions in Excel formulas.
As long as you type a date in a format that Excel recognizes: 8/22/2011, 22-August-2011, or August 22, 2011, for example, Excel will interpret the date as a serial number, and you can use formulas to find important dates. For example, you can then use formulas to subtract one date from another.
If you're not sure whether you typed a date in a format that Excel recognizes, do the following:
Go to the Home tab on the ribbon.
Click the arrow on the Number group, select Date, and then pick a format.
Link: Dates and formulas - Figure out dates by using formulas in Excel 2010 - Microsoft Office