Find out end date of DSL Young contract

Hallo,

I have a simple question I cannot figure out by looking at any of the documents or the web:

When does the 2 year period of my DSL conctract ends?
Story:

• I signed the "Auftrag" at a O2 shop on the 06-04-2017
• I received the Router around 15-05-2017
• After several cancellations and techniker-not-showing appointments, the technician finally came some time in august: the first bill I have is from 18-08-2017
• In the first bill it says "Aktivierungsdatum: 15.05.2017" .. and they actually charged me 50% of the tarif from 15.05 till 14.08 when I did not have any service yet. (I leave this for another thread)

I would expect that the 2 years period finishes on the date when I started having service: around the first bill in august.

Please I need to confirm this point as soon as possible.

Thanks