Purchase Order No. should be restart on every financial year(april 2012-march 2013)

Question

Hi friends,
i have a requirement in which i have a,
Field name PO No. & its format is like(DSCL-mm-yy / Kit 01)
in which DSCL is fixed
mm-stands for month
yy-stands for year
kit is the name of item,it is fixed
01 is the PO Number which will get incremented by 1 as the PO are generated.
my query is how to restart the PO number once the Current financial year(april 2012-march 2013) gets close and new begins.
how to get the following pattern

pl reply!

Regards,
Aamir