Thx @rwoolhou and @abenhadj. My first question is answered, so this discussion is solved.

The commands (rpsetvar 'flow-time ##) and (rpsetvar 'time-step ##) are working where ## is the required value.

@rwoolhou: Can you clarify why it is risky to use this, pls? Would it make a numerical difference to the results if I change the starting time with the above commands during starting a new simulation?

Thx for the help.

Best regards,