More than that; you're forgetting about locale specific DST rules. It is increasingly common for DST to be observed at different times in different countries. Then there are also places (such as Arizona) that don't observe DST at all as well.

This is a mess that can only be solved by isolating 'meeting' and 'appointment' times from local time. For local time you might specify X on date Y, but the software should, at the time of input, convert that to an actual hard time (in UTC).