With this function you can get the number of weeks between two dates. This value is always positive, and incomplete weeks will be returned as a fraction.
|date1||The first datetime.|
|date2||The datetime to compare it to.|
diff = date_week_span( date_create_datetime( 2011, 9, 15, 11, 4, 0 ), date_current_datetime() );
This would set "diff" to the number of weeks between 15th September 2011, 11:04.0 and the current date and time.