This is actually the best approach.

Obviously they are getting timezone information otherwise the app could only display whatever time the user entered in.

If you want to sort things by the actual time, it’s simple and performant if all of the times are in the same timezone, and UTC would be the standard one to use. Pushing the timezone calculations to the client makes sense because the UTC time is correct, it’s just a matter of displaying it in a user friendly way, ie. show the time in the user’s timezone.