Calculating Fractions of Years

One of the types of data that Excel allows you to store is, of course, dates. At some point you may wish to perform some calculations with the dates in your worksheet. It is not uncommon to need to figure out the percentage of a year represented by the difference between two dates. Excel allows you to calculate this easily using the YEARFRAC worksheet function. This function is part of the Analysis ToolPak provided with Excel.

To use the function, all you need to do is provide two dates and a value that specifies how Excel should calculate the fractional year:

=YEARFRAC(DateOne, DateTwo, Basis)

The dates used by YEARFRAC can be either static dates, or they can be references to cells that contain dates. The Basis value ranges between 0 and 4, with 0 being the default. The following are the different meanings for the Basis:

You should note that if the YEARFRAC function does not work on your system, it means you have not installed or enabled the Analysis ToolPak. To enable it, follow these steps:

1. Choose Add-Ins from the Tools menu. Excel displays the Add-Ins dialog box. (In Excel 2007, click the Office button, then click Excel Options, and finally click Add-Ins.)
2. Make sure the Analysis ToolPak option is selected.
3. Click on OK.

If you did not see an Analysis ToolPak option in step 2, it means that you did not install the option when you first installed Excel. You can rerun the Excel Setup program and choose to install the option. You must then enable the add-in, and you can use the function.