Im not an expert on these kind of formulas but have based a date based calculation on someone else’s formula.If it helps this is the formula;
[=FD(ADM('{TICKET DATE}',-216),'dd-MMM-yyyy')]
What thats doing is taking ticket date and subtracting 216 months (18 years) to give a date of birth for 18 year old for age check message.
=FD being format date
I guess ADM is something like ADD DATE MONTH where -216 is the number of months to add the minus making it subtracted.
‘dd-MMM-yyyy’ being the format output so with that today would be 09-JUL-2015