Budget Control - Is need-by date used?

 We are considering implementing budget control; however, it is not clear from the documentation which "date" is used to check funds.  We would want to check budgets by quarter.  If I enter a purchase order for software maintenance that covers a year, can I enter 4 lines using a need-by date in each quarter and pass budget checking?  My basic question:  what dates are used (on requisitions and purchase orders) for comparing against budgets when budget control is enabled?