Query Expression - Age as of a specific date, not SYSDATE

Hello,

I'm trying to calculate employee's age as of 01/01/20XX, using PS Query Manager.  I am able to get their age as of SYSDATE using this expression:  MONTHS_BETWEEN(SYSDATE, B.BIRTHDATE)/12 ... I've tried a few variations to try to calculate the age as of 01/01/2018... but I keep getting an error.  What is the proper way to write the expression where it uses a specific date, rather than SYSDATE.

Thanks in advance,

Jennifer