Calculating Months for Billing Purposes

Olga keeps the books for a private school. For each student enrolled, she has an entry date and a drop date. She needs to figure out how many months to bill each student. If the student was in class for at least five days, that month should be included in the billing. If less than five days, they are not billed for that month. She also needs to exclude holidays and weekends.

Integral to any solution to this problem is going to be the use of the NETWORKDAYS function. This function, as described in other ExcelTips, is part of the Analysis ToolPak. Assuming you have the ToolPak enabled, the function will calculate the net number of work days between two dates. It takes into account weekends and, optionally, holidays.

So, assuming you have the student's entry date in A1 and the drop date in A2, the only thing you need to do is set up a list of holidays. You can do that by simply starting to enter the holidays in a range of cells. Enter one date per cell, and then select the range. Define a name to refer to the range, such as MyHolidays.

You can then use a formula such as the following:

=(NETWORKDAYS(A1, DATE(YEAR(A1), MONTH(A1) + 1,0), MyHolidays) >= 5)
+ (NETWORKDAYS(DATE(YEAR(A2), MONTH(A2),1), A2, MyHolidays) >= 5)
+ DATEDIF(DATE(YEAR(A1), MONTH(A1) + 1, 1), DATE(YEAR(A2),
MONTH(A2), 1), "m")

The formula is quite long, and bears some examination. Note that besides the NETWORKDAYS function, it also uses the DATEDIF function, which is used to determine the difference between two dates and return the interval in different ways. In this instance, it is used with the "m" parameter, which means it returns the interval as a number of months—exactly what is needed by Olga.

The first part of the formula (the first use of the NETWORKDAYS function) is used to determine how many days there are between the entry date (in cell A1) and the end of the month in which the entry date occurs. If this value is greater than or equal to 5 (Olga's cutoff), then the value 1 is returned, since this counts as a single billable month.

The next part of the formula (the second use of the NETWORKDAYS function) is used to determine whether there are at least five class days in the month in which the drop date occurs. If so, then the value 1 is returned, again because this is a billable month.

The DATEDIF function is then used to return the number of full months between the entry month and the drop month. What you end up with is a count of the number of months that should be billed for the student.