Yes, that is possible.

Do you have a continuous pass or a flexi pass?

I have a global pass for 15 days within 2 months

Ok. Have you planned to use all 15 travel days? 

The thing is that when you activate your pass, then the start date of the validity of the pass is automatically made a travel day. This means that the 31/8 will automatically be a travel day. If you don't plan to use all 15 travel days, then this is no problem.

If you need all 15 travel days in September, then you need to delete the travel day on the 31/8 and answer "no" to the question if you want to deactivate you pass.  The your pass should be active from the 31/8 but without any travel days used.

Ok that’s weird. But it won’t be a problem. So far I’ve only planned for 12 travel days