To caluculate cal month from fiscal year using formula in update routine?

HI,

i want to caluculate calender month from fiscal year?

In detail i want to caluculate fiscalyear in quater wise, i created a Zdate character feild, and in update rules i want to write formula for cal month from fiscal year (pppp->yyy mm)and then i want to caluculate (yyyy mm ->Q).

can anybody tell me how to convert that zfeild date