Root cause: we were creating a `Date` that did not exist in my timezone
(Santiago, Chile) because of DST. The JS engine decided to pick the
previous day, so I ended up with two times 5 September.
Fix: introduce a `DateOnly` object that internally uses UTC (to prevent
DST issues). Other parts of the system still use `Date`, which
probably should be refactored to use `DateOnly` as well (maybe in an
upcoming PR?). In the meantime, we provide a function to export a
`DateOnly` as a `Date`, using noon as the time instead of midnight
(because there are no DST changes at noon AFAIK).
We tested this manually (see the attached screenshots), but didn't find
a suitable place for automated tests.
port 1.6
