A common type of date calculation determines the number of days between two dates. For example, you may have a financial worksheet that calculates interest earned on a deposit account. The interest earned depends on the number of days the account is open. If your sheet contains the open date and the close date for the account, you can calculate the number of days the account was open.

Because dates are stored as consecutive serial numbers, you can use simple subtraction to calculate the number of days between two dates. For example, if cells A1 and B1 both contain a date, the following formula returns the number of days between these dates:

=A1-B1

Excel automatically formats this formula cell as a date rather than as a numeric value. Therefore, you will need to change the number format so that the result is displayed as a nondate. If cell B1 contains a more recent date than the date in cell A1, the result will be negative.

Sometimes, calculating the difference between two days is more difficult. To demonstrate, consider the common fence-post analogy. If somebody asks you how many units make up a fence, you can respond with either of two answers: the number of fence posts or the number of gaps between the fence posts. The number of fence posts is always one more than the number of gaps between the posts.

To bring this analogy into the realm of dates, suppose that you start a sales promotion on February 1 and end the promotion on February 9. How many days was the promotion in effect? Subtracting February 1 from February 9 produces an answer of eight days. Actually, the promotion lasted nine days. In this case, the correct answer involves counting the fence posts, not the gaps. The formula to calculate the length of the promotion (assuming that you have appropriately named cells) appears like this:

=EndDay-StartDay+1

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