In 1st 2 formulas, you checked for ...AND(ISNUMBER($I2); ISNUMBER($K2))...
This expression returns True, when both $I2 and $K2 have numeric values. For all other cases an empty string is returned.
3rd formula worked OK for me! I think for you at least one of cells $I2 or $K2 was not empty (e.g. there was a space entered - I tested the formula, and your 3rd formula returned error in case into any of those cells as space was entered).



Btw, when you want the formula to be ideal, you have to check the result date for it being February 28th or the source date used for it being February 29th.. When this is the case, then formula must return 0th day of March instead, so depending the year being leap one or not, either 29thor 28th day is returned!