Hi Asklepios,
Yes, our licenses start from the date of purchase. I was under the impression that it was written somewhere in the webpage but I cannot find it now :/
Anyhow, I extended your license/s by 30 days as compensation
You may need to apply the license again to see the changes. To do so either activate the same code again, or log in to your Avast account (my.avast.com) from the application and click in "Use this license", or you can as well download them from the Avast account portal.
Cheers!