The way I see it is you are deciding between making the instant very clear or making their "time of day" clear.

But IMHO "time of day" is less important because 1. People in the same time zone still have different schedules so it is an approximation anyways. 2. If you know what instant is being discussed it is very easy for each person to relate it to their personal schedule.

So I agree that switching costs aside using UTC would be better overall.