A problem with the future is that it is hard to predict. Define whatever format you want, and there's always the possibility of a politician changing what the

A problem with the future is that it is hard to predict. Define whatever format you want, and there's always the possibility of a politician changing what the local time is. Historically, the offset against UTC has changed by a few minutes, hours, or days. Is daylight savings used? When? It is not even certain that it is the same calendar that is used, or that there are 24 hours per day, when the day starts, which months there are, or which month is the first of the year. If the time zone changes in the future, so that the specified local time no longer corresponds to the same time in UTC, then the time of the event has changed, even if the organizer still specifies 6 pm.