As you know, Excel is great at arithmetic.
Well, even when it comes to dates, Excel allows you to use simple arithmetic to calculate the number of days between dates buy subtracting one date from another. This is possible due to the way that Excel stores dates.
Each date is equal to 1 and part of a day (i.e. hours and minutes) are stored as a decimal. To Excel, the date July 10, 2009 is actually a number (40,004). If you format a cell containing a date to a number you will see it’s decimal equivalent. The number represents the number of days since January 1, 1900.
With this information, it takes just a simple subtraction formula in Excel to determine the number of days between two dates.
You subtract the oldest date from the newest date. If the dates are contained in cells your formula might be =B1-A1.
If you want to count the days between a certain date and today, you can use the TODAY function in your formula. For example,. =TODAY()-A1
You can follow this link to determine the number of months and years between two dates.
You can also extract the year, month or day from a date which can be useful in formulas.