First, I assume that A and B are numeric columns with the date stored as the number of seconds since midnight, January 1, 1904, as required by JMP. You cannot use character string representations of dates for this computation.
Second, see the Date Difference() function.
So you want 1 if A >= B? What result do you want if the comparison is false?
You could use just Date Difference( A, B, "day" ) > 0 if you want 1 when it is true and 0 when it is false.