An example of what I'm trying to do is I enter the date 12 / 25 / 1984

I need to add every digit like 1+2+2+5+1+9+8+4

which would equal 32, but i need to keep adding the digits until I get a single digit answer

I do not know how to separate 12 into 1 + 2 or 1984 into 1+9+8+4