#NUM! error - how do I make it Zero?

I have a document where I need to work out the difference between two dates.  this will be duplicated up to 1000 times, and needs to be dynamic for adding different dates in.  but I am getting a NUM value due to the example below because the start date is after the 22 years of age cell.  I need it to say Zero in this instance, as further in the spreadsheet I need to add multiple numbers up.  Cell Y6 being one of them.

CELL KEY

DOB = V6

Start date = W6

22 Yoa - X6

Years to pay = Y6   #NUM! value one

The formulas I have used are as follows:

22 yoa =(DATE(YEAR(V6)+22,month(V6),DAY(V6))

Years to pay =DATEDIF(W6,X6,"y") 

Any help much appreciated, Im scratching my head, and really dont want to have to work all these numbers out by hand.

Thanks in advance