Sometimes I should really read more carefully when the always excellent MDN says…
The Date.now() method returns the number of milliseconds elapsed since January 1, 1970 00:00:00 UTC.
… which means, when read carefully, since 1970/01/01 UTC *and not* since 1970/01/01 in UTC.
Other docs, namely the official specs rather put it this way:
The now function return a Number value that is the time value designating the UTC date and time of the occurrence of the call to now.
It is hard to bear in which enchanting beauty this sentence shines! Grrr ....
Completely logical, isn't it? I'm pretty sure I got it once and for all. Well, at least until tomorrow morning.