A <time> and <datetime> element exists in HTML, but why don't an element exists that will format a date and time written in a specific format to the format of what the user want? I'm talking about automatic TZ conversion.
It's been a while since I did any frontend work, but I recall that the Date Javascript object does do that. I was able to just send a UTC timestamp from the backend, then do something like (I've not tested this right now)
new Date(timestampMs).toLocaleString()
and it just worked. Correct timezone and locale. Not sure why that simple approach isn't more widely used
For example, sometimes I mostly care about when, in relation to my local time (and in particular how long ago) something was written; other times, the local time of the event happening is relevant too (e.g. “did this happen before or after markets close in the country where it happened”).