I believe the way it works (and anyone feel free to correct me) is: 365 days after you register the airtime (phone the number and punch in the serial) any unused minutes are gone and not recoverable. However, whenever you add more airtime the expiration for ALL airtime, including remaining balances is extended 365 more days.

So if you register 100 dollars of airtime and use 90 dollars up over the year, when you register another 25 dollars on the 355th day the remaining 10 dollars balance is good for another 365 days after that, along with the new 25 dollars you just added to your account.