When you reach a tier level you get a minimum of 12 months at that level. If you reach that level before your year end date you gain the tier level for the rest of your year and for 12 months after that. The card expiry date is always the month after your status expires to allow for your card to be renewed or replaced with a lower tier card if you fail to re-qualify.

As an example my year end is 8th March, I achieved Silver last November, therefore my Silver status lasts until March 8th 2014 ( rest of my year to March 8th 2013 plus 12 months) and my card expires April 2014.

To answer your lounge access question, this is based on the card expiry date, so you will gain access until your card expires.

I hope this helps.