What is your definition of "month", sometimes it's sufficient to subtract the two dates--which produces the elapsed time in days and then divide by 30.
( Get ( CurrentDate ) - contract_end_bup ) / 30
Other times, that will not meet the needs of your system.
Let ( [ M1 = Month ( contract_end_bup ) ;
Y1 = Year ( contract_end_bup ) ;
M2 = Month ( Get ( CurrentDate ) ) ;
Y2 = Year ( Get ( CurrentDate ) )
12 * ( Y2 - Y1 ) + M2 - M1
) // Let