Month Formula

If you have any blanks or cells that do not have a date in them, you will get an error. To account for these cells, wrap the MONTH function in an IFERROR to generate a 0 (zero) for a cell that would otherwise throw an error. Because it is replacing the error with 0 and there is no month 0, these cells will no longer throw the error and will not be counted in the formula.

=JOIN(COLLECT(Date:Date, Date:Date, IFERROR(MONTH(@cell), 0) = 1))

=COUNTIFS(Date:Date, IFERROR(MONTH(@cell), 0) = 1)

The error is coming from the MONTH function itself. The MONTH function is looking for a date. Referencing a blank cell is the same as saying

=MONTH()

or a cell with "abc"

=MONTH("abc")

Which obviously will not work.

With the MONTH function throwing an error, the formula as a whole will break and in turn throw an error.

.

My explanation using "Month 0" isn't entirely accurate.

The MONTH function produces a numerical value which is what we are comparing to the number 1. The IFERROR is telling the formula that if the MONTH function throws an error (as explained above), just produce the number 0.

Since 0 is not equal to 1 but is still a numerical value that can be compared to other numerical values, it allows the rest of the formula to continue working while excluding it from the count. 

The reason I use the number 0 is because you will never look for month (or year for that matter) 0.