The YEAR() function wants a date as input. If you subtract two dates you will get some small number of days which the YEAR() function will then interpret as a date. So if the difference in days is less than 365 then result will be a date in the year 1960 so applying the YEAR() function to it will return 1960.
How do you want to define the difference in years? Do you just want to subtract 2015 from 2019?
number_years=year(admdate)-year(svcdate);
Do you want to take the difference in days and divide by an average number of days in a year?
number_years=(admdate-svcdate)/365.25 ;
Or perhaps you want to use the INTCK() function to count for you.