How sum Dateformula fields - Another question!!

Hello again, I'm doing operations with dates and, for example, I've two Dateformula fields. I've created a Dateformula variable and I want that it shows the sum of the other two dataformula fields, but I can't. I put:

prob:=((FORMAT("Field1"))+(FORMAT("Field2")));

Field1 is 3D
Field2 is 4D

But the result is that prob shows 3D4D, it should be the sum of that days, 8D. How can I do that?.
Thanks in advance.