Determining Differences Between Dates

When you are programming Excel macros, you should know that dates are stored internally, within variables, as serial numbers. The serial number represents the number of days elapsed since a starting "base date," specifically since 1 January 100. This means that you can perform math with the serial numbers, if desired. You can, for instance, find the number of days between two dates by simply subtracting the dates from each other.

If you want to get fancier in your date calculations, you can use the DateDiff function. This function allows you, for instance, to determine the number of weeks or months between two dates. In order to use the function to find this type of information, you would do as follows:

iNumWeeks = DateDiff("ww", dFirstDate, dSecondDate)
iNumMonths = DateDiff("m", dFirstDate, dSecondDate)

The first line determines the number of weeks between the two dates, and the second determines the number of months between them.

Remember that the DateDiff function is a macro (VBA) function, not a worksheet function. Excel handles a range of dates in worksheets that begin with January 1, 1900. In VBA, however, dates can begin (as already noted) in the year 100. That means that macros can handle a much larger range of dates, including dates prior to those handled natively by Excel.

Even so, you need to be aware that historical dates can have weird quirks associated with them. For instance, in Great Britain the calendar went from September 2, 1752, to September 14, 1752, skipping all the days between. This was done in conjunction with Great Britian adopting the Gregorian calendaring system. Quirks like this can vary by city or country, and DateDiff doesn't take them into account in its calculations.

ExcelTips is your source for cost-effective Microsoft Excel training. This tip (9046) applies to Microsoft Excel 2007, 2010, 2013, 2016, 2019, 2021, 2024, and Excel in Microsoft 365. You can find a version of this tip for the older menu interface of Excel here: Determining Differences Between Dates.