Hi Paul,

07a4dd8e6a introduces a change in the behavior of `parse-time-string':

(parse-time-string "2020-12-12")
  [before 07a4dd8e6a] => (nil nil nil 12 12 2020 nil nil nil)
   [after 07a4dd8e6a] => (0 0 0 12 12 2020 nil -1 nil)

The current docstring seems to suggest that the current implementation
should return (nil nil nil 12 12 2020 nil -1 nil).

I suggest the attached patch to revert to the previous behavior while
still having the benefits of the new implementation.

The new behavior causes an annoying bug in Org-mode: re-scheduling a
headline from the calendar introduces a default time value ("00:00")
even when there is no time in the timestamp.

WDYT?

Thanks,

-- 
 Bastien