How do I write an Excel formula to compare year-to-date to prior years & also account for leap years? - excel

Question

I’m trying to compare a measure as of today through the same day and month for the prior 4 years (e.g. through June 6 of 2016, 2015, 2014, etc.).
For each year, I decided to count the number of days since the beginning of the year, and sum my values through that number of days for each year.
To identify whether a date should be included in the year to date comparison, I used the formula where my date is in cell A1:
=IF((A1-DATE(YEAR(A1),1,1)+1)<=(TODAY()-DATE(YEAR(TODAY()),1,1)+1),1,0)
I’m looking for a way around the issue of the extra day added to leap years. In other words, after February 28th, the day count will always be off by one in a leap year, and trying to use Februrary 29th in a non-leap year will return an error.
I’d like to adjust this formula, but I’m open to using a different function & formula if it gets me the right results.

Answer 1

you can check any information about February, 29th. If an error occurs, you know its no leap year. Catch that error with =IFERROR(;).

Answer 2

Assuming a table structure like this:
A:Date | B:Value
----------------------
01/01/2016 | 0
01/01/2015 | 1
01/01/2014 | 2
01/01/2013 | 3
01/01/2012 | 4
Formula
To - for example - calculate the average of the previous four (excluding the current) years on January 1st (today is 01/01/2016):
=SUMPRODUCT(
(MONTH(A:A)=MONTH(compare))*
(DAY(A:A)=DAY(compare))*
(YEAR(A:A)>YEAR(compare)-5)*
(YEAR(A:A)<YEAR(compare))*
(B:B)
) / (
SUMPRODUCT(
(MONTH(A:A)=MONTH(compare))*
(DAY(A:A)=DAY(compare))*
(YEAR(A:A)>YEAR(compare)-5)*
(YEAR(A:A)<YEAR(compare))*
1
)
)
Result
For the above example, the result is 2.5
Explanation
To select only those rows representing the same month and day:
(MONTH(A:A)=MONTH(compare))*(DAY(A:A)=DAY(compare))
To select only those values from the previous 4 years (excluding the current):
(YEAR(A:A)>YEAR(compare)-5)*(YEAR(A:A)<YEAR(compare))*
The actual values we are interested in:
(B:B)
Divide by 4 for the average over the last four years. This assumes there is no missing data which might be an issue. You could use another SUMPRODUCT (replace B:B with 1) to count the number of resulting rows and divide by that number to handles this case. This seems to be rather slow, but it works.
Note
For performance reason you should not use A:A (a full column) in the formula, just use the actual range you need, which will likely be much faster.