Hi David Vanechaute,
As Martin mentioned, if you want to see specific date on the report header you need to add new parameter to report contract and pass it through to the report, it could be current session date time or new field on the dialog or something else.
However, I don't see any other way because again, as Martin mentioned, there is no "client" session when report is executed and don't forget that you can create recurrent batch for example...