The 15 nights post on your anniversary date and since sstatus extends thru the next year after you qualify you always have silver status if you have the card.
For example a member with no nights gets the card on April 1, 2011
15 nights post April 1, 2011
Silver status extends thru end of 2012
15 nights post on April 1, 2012
Silver status extends thru end of 2013
15 nights post on Aprill 1 2013
and so on.