> To see if I understood this change: now you will use the owned (incremented here) outputTimeStepIdx_ instead of the timer's currentStep() to choose the file name? So that even if I want output_interval=7 I will get ...00001, ...00002 and so on instead of ...00007, ...00014 etc? I do not like this change, but I can accept it for now to get this merged.

yep. because 00001 IMHO means "Second (or first, depending what you want) report step"