Timestamps printed with --time-format=iso are not valid ISO 8601:

    # wtmpdb last --time-format=iso | grep -wm1 boot
    s-reboot system boot  6.1.0-50-amd64   2026-07-17T18:15:14+0000  - still running

This is invalid, because there's no colon in the timezone offset.

In contrast, util-linux's last(1) did it right:

    # last --time-format=iso | grep -wm1 boot
    reboot   system boot  6.1.0-50-amd64   2026-07-17T20:05:09+02:00   still running

Hi Jakub,

I'm not au fait with ISO 8601, which appears not to be a free standard,
but from a brief web search it seems the +HHMM form might be a valid
variant and indeed, that which is produced by strftime(3) %z according
to its man page.  Would you mind raising an issue upstream where you can
argue the point directly with the upstream maintainer?

Regardless of its correctness, while I agree for continuity it would be
nice if this matched util-linux, we missed the boat for this in trixie
and neither the version of last in src:sysvinit or src:acct supports the
iso option so I think the case to patch in Debian would be weak. Do you
know of any downstream tooling depending on this behaviour?

Thanks

Andrew