Nice article. For your rule of thumb section there is a third case. For a future event that must happen x number of seconds/minutes/hours/days into the future you want to store it as UTC and not wall time.

For example, the bypass valve on the boiler must be opened in 30 hours. You want that as UTC instead of wall time as you don't want a DST change to add an extra hour to that as you'd get a pile of scrap instead of an intact boiler.