I have been thinking about doing something like this, but I seriously dislike that the time zones need to be handled explicitly in the changeset functions. That is guaranteed to introduce a bug pretty much every time.
I agree that a hidden field is probably the way to solve this. In order to have a regular :utc_datetime work, the hidden field would need to contain the datetime with the time zone together, rather than be a separate field with just the timezone. I should probably give this a try when I have recharged my mental capacity 
I’m mostly just surprised that nobody seems to have solved this properly before.
