You have an incorrect syntax: To use your formula layout you need something
like:
=(YEAR(DATE(2015,10,16))-YEAR(DATE(2009,5,11)))*12+MONTH(DATE(2015,10,16))-MONTH(DATE(2009,5,11))
There are however easier ways to do this, I'm sure others will advise.


"Penny32" wrote:

have tried different formats for the dates, always comes up the same. have
formated in date/time format,
=(YEAR(10/16/2015)-YEAR(05/11/2009))*12+MONTH(10/16/2015)-MONTH(05/11/2009)=
1/0/1900.....what is wrong here?