The expiation date shows the end date of your annual plan (the end date of a 365-day period from the moment of purchasing the subscription).The expiration date of a 30-day period shows the date when you will receive an invoice for an overage (if there is an overage). In case you don't have an overage, the next 30-day period will start.
​Please note: the invoice for an overage must be paid within 10 days after its issuance or else your subscription will be blocked.