This PR fixes the `HSub PlainTime Duration` instance, which had its
operands reversed: it computed `duration - time` instead of `time -
duration`. For example, subtracting 2 minutes from `time("13:02:01")`
would give `time("10:57:59")` rather than the expected
`time("13:00:01")`. We also noticed that `HSub PlainDateTime
Millisecond.Offset` is similarly affected.
Closes#12918
