2023-09-17T18:31:34,607673525-04:00  ISO-8601.  date -Ins
2023-09-17 18:29:41.036415703-04:00  RFC 3339.  date --rfc-3339-ns
    BUT RFC 3339 doc says that these have T and end with EITHER ± OR Z
Sun, 17 Sep 2023 18:32:54 -0400      ISO-5322.  date -R   Seems to be only time offset without :

Internet date/time format

UNIX "date" doesn't seem to like the *Z formats.
what is the format with Z suffix?