For past timestamps, arguably a UTC record of when it happened is sufficient as converting it for display in a specific timezone is a fixed, and no longer mutab

For past timestamps, arguably a UTC record of when it happened is sufficient as converting it for display in a specific timezone is a fixed, and no longer mutable transformation. For a future looking time, it's not sufficient, because e.g. DST rules might change, timezones might change (and there's usually at least several occurrences a year of some timezone or DST rule changing somewhere in the world). If you have a reservation for the concert hall from 6pm next September, you don't want to show up there with the previous show still occupying the venue just because your government abolished DST in the year in between.