[tz] make rearguard_tarballs fails on macOS

[Robert Elz]

    Date:        Fri, 16 Oct 2020 20:02:41 -0700
    From:        Paul Eggert <eggert at cs.ucla.edu>
    Message-ID:  <c49c9641-9e74-a378-e3d2-62826072a403 at cs.ucla.edu>


  | macOS uses FreeBSD utilities, right? According to the FreeBSD man page
  | server, FreeBSD 'touch' has supported -d since FreeBSD 10 (2014).

NetBSD has a -d option to its touch as well, but it expects a "human"
format date & time, if the usage was:

	touch -md '2020-10-12 22:53:00Z' file

it would work, but:

	touch -md 2020-10-12T22:53:00Z file
	touch: Could not parse `2020-10-12T22:53:00Z'

With the 'T' in there, it is no longer a "human" format, but a
standard one, and isn't recognised - I guess I could look at fixing
parsedate(3) to handle that, but I'm not sure it is really warranted,
parsedate() is a horrible ugly chunk of barely fuinctional code, and
your updated patch will work fine.

kre