So.. is this a user error.. that elisp is not valid (i.e. nil not allowed in that list)?
So my second email in this thread was a false alarm then.
This works:
(apply #'encode-time
(mapcar (lambda (el)
(or el 0))
(parse-time-string "2018-09-05")))
But the first report on consistent timer errors in Messages buffer still holds.