This is its actual behaviour and it's the right thing to do, as it's
consistent with Now() -- even though the documentation wrongly stated
otherwise (since 324ab5e2db
).
Also add a unit test checking that UNow() == Now(), except for the
milliseconds.
See #14148.
Closes #18524.
Closes https://github.com/wxWidgets/wxWidgets/pull/1594