Amith,
The names of these dates Z1, Z2, and Z3 that you have used in your query do not seem to me to be a good match with names that I commonly see in SAP. Can you please provide a screen shot of Transaction ME23N, which displays an example Purchase order; where Z1/Z2/Z3 dates are displayed and highlighted?
'AB' usually refers to the order acknowledgement date. This date is not calculated, but is entered manually by a user, or sometimes by an IDOC through an EDI Transaction.
Best Regards,
DB49