What is error code 504?

I am attempting to use DATEDIF. Error code 504 is what I get. I am attempting to determine the ages of 200 plus women that were allowed to vote for President of the U.S. for the first time in 1920. The women were all born in the 1800"s, and they voted Nov. 4, 1920. I did verify that generate # value is not checked. Any help to get me in the right direction is greatly appreciated.
thanks in advance to anyone that has the time.

Error Codes in LibreOffice Calc.

The mistake appeared to be with the end date; is was not formatted the same as the start date. The second possibility was that I had a period after the end date not a comma, but that may have been alate mistake as I was attempting to make changes. Thanks for providing a location for the error code.