Calculating Differences in Months using Non-Standard Date Values

Ian needs to enter the chronological age of a pupil in the first column, in the format 9.11 for nine years and eleven months, the reading age in the next column (say, 10.6 for ten years and six months), then calculate the difference in months in the third column, which in this case would be 7. The difference always needs to be shown in months, so it should show a gap as 14 months instead of 1.2 or 24 months instead of 2.0. Ian's at a loss as to how to put together such a formula.

The trick is to simply convert to a common set of units, in this case months. So, for instance, 10.6 would be 12*10+6, or 126 months. If you do this type of conversion to both the chronological age and the reading age, then you can do whatever subtraction you need in order to determine the difference, again in months.

There is a big, big caveat here, however: How you enter your non-standard dates into the worksheet is going to have a huge bearing on how you work with that data. The problem is best illustrated by considering a date value such as 10.10, meaning an age of 10 years and 10 months. If you enter this value directly into a cell, Excel will parse it as a numeric value and change it, automatically, to 10.1 because the trailing 0 is (to Excel) insignificant. Unfortunately, this value is indistinguishable from 10.1, meaning 10 years and 1 month.

There are two possible solutions. If you want to enter numeric values, then you should make sure you always include a leading 0 for the months. Thus, you would enter 10.01 or 10.06, not 10.1 or 10.6.

In Ian's problem statement, however, he specifically uses 10.6 as an example, which means that leading zeros are not being entered. In this case, you must make sure that what you are entering is parsed as text by Excel. In other words, make sure you format those date-entry columns as text before you start entering the dates. In that way, Excel will show 10.10 as just that, with the trailing zero.

All of the solutions you use to do the calculations assume that you are entering dates with the leading zero for months or that you are entering the dates as text values. In all instances, let's assume that the chronological ages are in column A and the reading ages are in column B. If your dates are formatted as text, you can then use a formula such as the following in column C:

=INT(B1)*12+MID(B1,FIND(".",B1)+1,2)-(INT(A1)*12+MID(A1,FIND(".",A1)+1,2))

It returns a positive value if the reading age is greater than the chronological age, or a negative value if the reading age is less than the chronological age. You can't use the formula if the dates are entered as numeric values because if the age is entered as 10.00 (meaning 10 years and 0 months), Excel still parses that value as 10. The formula then returns a #VALUE! error because the FIND function cannot locate a non-existant decimal point.

If you are using numeric-formatted dates (again, with leading zeros for months) then you can rely on either of these two formulas:

=12*(INT(A1)-INT(B1))+100*(MOD(A1,1)-MOD(B1,1))
=(DOLLARDE(A1,12)-DOLLARDE(B1,12))*12

The key is to be consistent in how your date values are formatted, use leading zeros for months if entering in numeric format, and use the proper formula based on whether you are working with text or numeric values.

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