My answers at each stage, provide a number of alternative versions of what appear to be the same thing - but aren't!
The question itself was essentially "How do I calculate the number of months between two dates?"
My first response to this question used the DATEDIF function as featured in this earlier post.
Assuming that the start date is in cell A1 and the end date is in cell A2, then:
will return the number of WHOLE months between the two dates.
My client then said that they wanted to always round up the number of months.
I then used the "MD" argument of a DATEDIF to identify the remaining days after calculating the whole months, so that I could use and IF statement to add 1 if this remainder was greater than zero:
Finally, it transpired that what was really required was the number of calendar months touched by the date range, e.g. if the start date was 31st January 2015 and the end date was 1st February 2015, the answer should be 2, as both January and February feature in the date range (with the same dates, our first example would return zero, as there are no whole months and our second would have returned 1, as we rounded the 2 days up to a whole month).
This required a completely different approach, by using the month function to pull out the month from each date, and the year function to pull out the year. The answer would then be the difference between the month numbers, plus 1, plus 12 * the difference in the years, i.e.
Depending upon your specific needs, any one of these formulae might be correct for your requirement!
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