Get the date out of the SQL database and convert it into a time stamp.
$sqltime = mktime(hour,min,sec,month,day,year); - you will probably need to do some string manipulation to get it into the right format.
Then get the current time.
$currtime = time();
Get the difference in seconds.
$difftime = $currtime - $sqltime;
Divide by the difference by the number of seconds in a week.
$numweeks = floor( $difftime / 604800 );
That should give you the number of complete weeks that make the difference.