just add the two dates to the resultset - means adding two dates to every row though. Could also include a new datasource which just has those dates - that's if you want to keep the calculation in the database.
It's probably easier if you pass the dates from the report as parameters then you can use those parameters in the header.
Could also just do the calculation again in the report.
Cursors never.
DTS - only when needed and never to control.