This is always such a ball of yarn, but working in a UI where both “absolute” and “relative to the locale of where it happened” are important contextual information, I’ve come to the conclusion that it is actually quite sensible to show both UTC and “local to where the event occurred” time at the same time, every time.

From my experience, implementation of time zone conversions is too often wrong. I work with many time zones, and strongly prefer either GMT+0 or “x hours ago”.