SQL & PL/SQL

Hi,

I'm writing a PL/SQL program and I'm facing the following problem:

I have a date D in the format 'dd/mm/yy' and I want to get the beggining of the quarter in which this date is located , and also the beginning of the semester in which the date is located. I want the beginning of the quarter and the beggining of the semester in the format 'dd/mm/yyyy' (which is the format of the date D).
For example, if my date D is 12/11/2007 , the beginning of the quarter is ,in the format 'dd/mm/yyyy', 01/10/2007, and the beginning of the semester is ,in the same format 'dd/mm/yyyy', 01/06/2007.

Thanks you in advance for your help.