Of course a short search on Interrail.eu and you could find this https://www.interrail.eu/en/plan-your-trip/rail-planner-app

If you buy a paper pass you have to select already in the purchase process when the Pass Validity starts can be any date within the next 11months :)
If you buy a mobile pass you have to load your pass in the App from the link above and you can decide within 11months after purchase when the Validity time should start :)

Enjoy your trip :)

Note that in many countries they run their own system and you have to report to THAT railway-sometimes even only at counter. Most newbees here do not seem to realise it is simply a system combined of some 100+ different railways and each has its very own special system-often quite burocratic. Railways in general here in EUR are not noted much for user friendliness, more for burocracy.

Thank you very much for your answers!