doc: remove mention of UT1 in representing time

UT1 was incorrectly specified as our time representation.  (UT1 is
astronomical time.)  We are not actually UTC either because we ignore
leap seconds.

Reported-by: Thomas Munro

Discussion: CAEepm=3-TW9PLwGZhqjSSiEQ9UzJEKE-HELQDzRE0QUSCp8dgw@mail.gmail.com