Hi Marc,
what you can do if you always want to select the current month is to change the input control to a customized one which just returns the current month, e.g. in oracle via sysdate.
Alternatively, you could change the OOTB report via the Jaspersoft studio and change the SQL so it uses the sysdate as a filter.
The best solution would be to use a new input control to allow the user (case statement in SQL) to choose whether the current date or the filtered date should be applied.
Regards
Georg