You could use: RELEVEDATE - JOINDATE to get the difference between them in days. To convert this into month you could use an approach like this one: How Do I Get the Number of Months Between Two Dates?
It seems the dates which you have is not in consistent and correct date format.
In your load script you need to convert "JOINDATE" and "RELEVEDATE" in date format.
alt( date#( JOINDATE ,'DD-MM-YYYY' ), date#( JOINDATE , 'DD/MM/YYYY' )) as JOINDATE,
alt( date#( RELEVEDATE ,'DD-MM-YYYY' ), date#( RELEVEDATE , 'DD/MM/YYYY' )) as RELEVEDATE;
LOAD * INLINE [
EMP, JOINDATE, RELEVEDATE
AA, 19-03-2011, 22-04-2014
BB, 23-03-2011, 21-03-2015
CC, 21/3/2012, 24-08-2016
DD, 20-03-2012, 23-06-2015
EE, 28-04-2015, 22-09-2016
Please find attached the example for same.
Test.qvw 148.2 K