Re: The format of time values in lisp/calendar/time-date.el.

[Richard Stallman]

    Was there an evolution in format from (HIGH . LOW), via (HIGH LOW), to
    (HIGH LOW MICRO)?

Yes.  Both (HIGH LOW) and (HIGH LOW MICRO) are valid now.
It is useful to accept (HIGH . LOW) as well.

    Shall I fix time-less-p and time-subtract to deal with (HIGH LOW
    MICRO) arguments as well?

Yes, please do.

    Should they deal with (HIGH . LOW) too?

It would not be a bad thing.

    What about the return value of time-subtract (and time-add): should it
    be backward compatible in the sense that it returns (HIGH LOW) if both
    its arguments are of that format?

Yes.