Timezones change, more frequently than you realize. Some countries even used to change when summer time started regularly. It's got better than it used to be afaik.
Also you need to take into account where the user is going to be. With future dates, time is relative. So at the very least you often need the time + the timezone of the location.
Book me a table at 8, means book me a table in Berlin time at 8, not in San Francisco time at 8.
Same with displaying future dates, you need the context of where.