bug#79078: date command: Relative dates fail with ISO 8601 time format and no zone

Tested on coreutils 9.5 (gentoo) and 8.32 (OpenSuSE).

There seems to be an interesting interaction in the code that 
parses relative dates in the --date switch; relative 
specifications used with ISO 8601 base dates can give incorrect 
results.

All tests below were run with a time zone of America/Los_Angeles.

The following date commands work fine:

    $ date -d '2025-01-20T12:00:00Z +180 days'
    Sat Jul 19 05:00:00 AM PDT 2025
    $ date -d '2025-01-20T12:00:00-0800 +180 days'
    Sat Jul 19 01:00:00 PM PDT 2025

However, omitting the time zone gives a surprising result that's 
only one day later, and at a different time:

    $ date -d '2025-01-20T12:00:00 +180 days'
    Tue Jan 21 01:40:00 AM PST 2025

Omitting the zone offset, but not using a relative expression, 
works:

    $ date -d '2025-01-20T12:00:00'
    Mon Jan 20 12:00:00 PM PST 2025

Using hours in a relative expression without a time zone gives a 
different kind of error:

    $ date -d '2025-01-20T12:00:00 +4320 hours'
    date: invalid date ‘2025-01-20T12:00:00 +4320 hours’

But including the zone shows that the +4320 hours isn't the 
problem:

    $ date -d '2025-01-20T12:00:00-0800 +4320 hours'
    Sat Jul 19 01:00:00 PM PDT 2025

Using a non-ISO date format works with relative dates:

    date -d '12:00 PM Jan 20, 2025 + 180 days'
    Sat Jul 19 12:00:00 PM PDT 2025

Note: I haven't tested with all of the other accepted date 
formats.

A final note: from my POV, it would be fine to just reject ISO 
8601 strings that omitted the timezone.  However, in that case an 
error more informative than just "invalid date" would be helpful 
to users.
-- 
    Geoff Kuenning   [email protected] 
    https://www.cs.hmc.edu/~geoff/

"There's a true schizophrenia where if you say to voters, you 
know, do
you think the federal government spends too much money and they 
should
spend less, they say yeah, absolutely.  Then you name specific 
things,
like Pell grants for students and they say, no, not that.  How 
'bout
NIH, medical research funding?  Nah, you really shouldn't cut 
that.
And pretty soon you've proved that what the American public is 
against
is arithmetic."
        -- Bill Gates, March 10, 2011