Dear prasad,
As per the standard process date planning can be maintained form top level to bottom level i.e: Project /WBS/Network/Activity levels, If we maintain Forward /Bottom up Scheduling types And planning profiles /planning methods , by defining the system as per the business requirement.
If we maintain bttom up planning system roll up dates from bottom level to top level hierarchy.
And forward planing method is maintained, it controls date from top to bottom level.
For Billing , only through Milestone Billing functionality we can maintain the billing project / or we can maintain the SD order Related Billing or Delivery Related Billing in genral process.
And for further any indepth process. go for Date Planning concept in SAP HELP.
Regards,
vrkumar