how could i get time-traveler-medal

You had to complete every quest (with a few exceptions, such as the Mount) from the Visitors event; the medal was obtainable from the final quest of the event, "Is This A Dream?". The medal cannot be re-obtained in the future (unless Nexon is weird and re-releases the event.)

Impossible; the timeline collapsed. Unless Dr. Bing is a retard and tries the same thing again, the event shouldn't repeat itself.