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 ge...@cs.hmc.edu
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
