Hi NAG,
I think the query is related to Milestone based Payments i.e. payments that will be done to the contractors or vendors based on Milestones (Purchase order involved here).
What you have mentioned speaks about Milestone Billing process related to Sales Orders.
@Avinash - Correct me if i am wrong.
I believe your requirement can be satisfied. Refer the link below,
Generating Dates from Milestones - Costs - SAP Library
Regards,
Gokul