For example having local time vs official time. This is what remote cities in western China are using. They've to use same time as east coast, but it makes no sense at all for them. So they plan everything around unofficial local time.
How do you know which was originally Thursday the earlier or later part? Or do you have to calculate it based off old UTC offset? What if you move across the globe? What if you talk to somebody online and they say they're doing smth on Thursday? How do you know which Thursday is theirs?
> Then again, people would develop a convention within days. A month, tops.
That's exactly my point -- the convention they adopt will most likely be to abandon UTC in favor of some local time. At which point, we end up with an ad hoc reinvention of timezones.