The vast majority of websites should be calculating dates in a server and merely presenting them to clients.

I have wondered why there isn’t a span style element which takes a UTC timestamp and presents it to the user in their preferred time zone. I even wonder if it could be done in private way so that JS cannot even access the value (made more difficult by layout and size).

Similarly a form element for date times could simply return the UTC from the local choice.

I am just wondering out loud and not an expert.