custom lists and custom formulas?

[nameruc]

i have a custom list of four quarters (Q1, Q2, Q3, Q4). I'd like to
have a formula in a cell that is able to calculate previous quarters in
the list.

For example: Q2 - 1 = Q1; Q4 - 2 = Q2.

I'd also like to be able to use years with the quarters. For example:
Q2 2006 - 3 = Q3 2005.

Is this possible? When i do that, i just get a #value error and i cant
find a solution! thanks.