What month is better to go to Tibet?

Nanjing to Suzhou is about 2 17 km. Timetable of different modes of travel from Nanjing to Suzhou: normal walking 6 km/h 1 day 12 hours 10 minutes by bike 20 km/h 10 hours 5 1 minute by cruise ship 28 km/h (still water speed) 7 hours and 45 minutes by motorcycle.