There're simpler ways

dob = DATE(YEAR(dob)-100, MONTH(dob), DAY(dob))
* Even simpler
dob = GOMONT(dob, -100*12)

>a = dob
>Set date LONG
>? Ctod( Alltrim(Str(Month(a)))+"/" + Alltrim(Str(Day(a))) + "/" + Alltrim(Str(Year(a) - 100)))
>
>dob = a
>