Indeed rev ticket usually have a duration from the date of the first sector flown. If you bought the ticket in Dec 2011 (date of issue) and flew first segment on Mar 2012 and the validity is 12 months, then your ticket should be valid for a last flight in Mar 2013. If the validity is only 6 month (this is often the case with CX I fares), then the final flight must be in Sep 2012.