I guess we could just put the lat/long and let people figure it out? But that still doesn't fix the time when the time zone does change... So using UTC is a simple fix
UTC doesn’t fix anything if you meant 6pm my local time on that day, come hell or high water.
Eg: Work at the local library is always 9 to 5 (again “local time”) and you mean precisely one hour after work lets up, regardless of whatever daylight savings or time zone changes occur between now and then.
You need to use either a naive date-time representation coupled with the additional context (“my time”) or use something like lat/long. Or you’re lucky and you live in or near enough to a major metropolis that has its own TZDB entry and it’s virtually impossible for your local time to diverge from that TZ, so you can use that instead (eg you live just outside NYC proper so you use America/New_York as your time zone).
Your example alludes to why there will always be a rift between UTC and local representations. Unless your system can understand "one hour after I end work for the day" correctly, any representation of that time will be subject to not just local timezones changes, but local work conditions for one or a few people (otherwise it can't be assumed a single time instance).
One is a a representation stripped of most ornamentation to be universally agreed upon, the other is a representation with many additional layers of context so it's locally understood.
There will likely always be a trade off between these, as a universal time is useful in all cases for only a small group of people, while a local time quickly loses relevance out of a local context.
One thing has nothing to do with the other. Timestamps are intended to simply record what the time is in a given place - not to telegraph expectations about local business hours.
China has one official time zone that covers five geographical time zones. Tell me, exactly how does saying "It's 6PM in China" tell me anything useful about the local library? You have to go to a local website and find out what time "in that part of China" the library is open. The timezone was never intended to express that.
