What I don't understand about it is, what the LEFT and RIGHT does and what the (A1,2) and (A1,3) do. And where do I place the code? In a new field or do I place it in the same field as the one with the Julian dates?
Using Excel, you can check what a function does by choosing Help On this Function. Eg for the LEFT function it says this:
LEFT returns the first character or characters in a text string, based on the number of characters you specify.
So left (A1,2) returns the two left most characters from the value in Cell A1, eg if cell A1 had the value 1000 in it, then =left(a1,2) would return 10.
You should put your formula adjacent to the value you are querying otherwise it will overwrite the value, and can't report anything. You would probably also create a circular reference, but that's another matter completely.