On the front, smack dab in the middle, the IDP says, Valid for One Year From: __________. At AAA, they fill in the date. Our AAA does dd mon yyyy, like in Europe. For example 01 Oct 2015.
The length of the validity has nothing to do with when you renew your state driver license or your passport.
As someone else mentioned, if you go back to Europe within that year period, you don't need to get another IDP. In our case, we each got one to begin on 01 Jun 2012 for a trip to France in June, 2012. We went back to the Netherlands and Belgium the following April, 2013. Because our IDPs didn't expire until May 31, 2013, we didn't need to get new ones.