An ‘Annual Subscription’ is one that runs from January to December of any particular calendar year. Therefore if you purchase an annual subscription product at any point during the year your subscription will run from the date of purchase until the 31st of December of that year.
All the releases that may have been published before the date of your purchase will already be included in the main work at the time of purchase. You will continue to receive any further updates that are published between the date of purchase and the 31st of December of that year.
For example, if you purchase an annual subscription product on 04/07/2008 then your subscription will run from 04/07/2008 until 31/12/2008. Any releases that may have been published between 01/01/2008 and 04/07/2008 will already be included in the main work when you purchased it on 04/07/08. You will also continue to then receive any releases that get published after 04/07/2008 up until 31/12/2008.