You must purchase it for the remaining days of the cruise. Sometimes they will let you buy 1 or 2 days into the cruise usually only on long cruises though. So if you sailed 15 days you could buy on day 3 but still need to pay for last 12 days.
11 cruises in 2012, 90 nights at sea visiting 15 countries
8 cruises in 2013, 56 nights at sea visiting 7 countries
Next cruise: None scheduled
PM me if you need a cruisemate for any Europe, transatlantic, or Panama Canal cruises
Date, ship, cost pp