GNU bug report logs - #50115
date command arithmetic involving the epoch produces "invalid date"

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Jeremy Cantrell <jmcantrell <at> gmail.com>
To: bug-coreutils <at> gnu.org
Subject: date command arithmetic involving the epoch produces "invalid date"
Date: Wed, 18 Aug 2021 20:43:01 -0500

Using `date --utc --date="..."` with a date specification that
decrements by years that should result in epoch=0
(1969-12-31T23:59:59+00:00) produces an "invalid date" error message.

The following commands should illustrate the problem:

Notice that the only difference between the starting dates is 1 second.
```sh
$ date -u -d "1970-12-31T23:59:59+00:00 - 1 year"
date: invalid date ‘1970-12-31T23:59:59+00:00 - 1 year’
$ date -u -d "1970-12-31T23:59:58+00:00 - 1 year"
Wed Dec 31 11:59:58 PM UTC 1969
```

The dates are only considered invalid if they fall on epoch=0:
```sh
$ date -u -d "1971-12-31T23:59:59+00:00 - 2 year"
date: invalid date ‘1971-12-31T23:59:59+00:00 - 2 year’
$ date -u -d "1972-12-31T23:59:59+00:00 - 3 year"
date: invalid date ‘1972-12-31T23:59:59+00:00 - 3 year’
```

Going the other direction seems to work:
```sh
$ date -u -d "1969-01-01T00:00:00+00:00 + 1 year"
Thu Jan  1 12:00:00 AM UTC 1970
```

It only seems to error when decrementing by years:
```sh
 date -u -d "1970-01-01T00:00:01+00:00 - 1 second"
Thu Jan  1 12:00:00 AM UTC 1970
```

It only seems to error when using --utc, because the following works
(my time zone is America/Chicago):
```sh
$ date -d "Wed Dec 31 06:00:00 PM CST 1970 - 1 year" +%s
0
```

Message #8 received at 50115 <at> debbugs.gnu.org (full text, mbox):

From: Jeremy Cantrell <jmcantrell <at> gmail.com>
To: 50115 <at> debbugs.gnu.org
Subject: Found the line where it is failing
Date: Wed, 18 Aug 2021 21:56:21 -0500

On line 2160 in `./gnulib/lib/parse-datetime.y`, there's this:

```c
          if (Start == (time_t) -1)
            {
              if (debugging (&pc))
                dbg_printf (_("error: adding relative date resulted "
                              "in an invalid date: '%s'\n"),
                            debug_strfdatetime (&tm, &pc, dbg_tm,
                                                sizeof dbg_tm));
              goto fail;
            }
```

This seems to indicate that, if the parsed value is -1, it's
considered an invalid date, but isn't -1 a valid date in epoch
seconds?

Passing --debug to the date shows the correct result, but claims it's invalid:

```sh
$ ./src/date --debug -u -d "1970-12-31T23:59:59+00:00 - 1 year"
date: parsed datetime part: (Y-M-D) 1970-12-31 23:59:59 UTC+00
date: parsed relative part: -1 year(s)
date: input timezone: parsed date/time string (+00)
date: using specified time as starting value: '23:59:59'
date: starting date/time: '(Y-M-D) 1970-12-31 23:59:59 TZ=+00'
date: warning: when adding relative months/years, it is recommended to
specify the 15th of the months
date: error: adding relative date resulted in an invalid date:
'(Y-M-D) 1969-12-31 23:59:59 TZ=+00'
./src/date: invalid date ‘1970-12-31T23:59:59+00:00 - 1 year’
```

Message #13 received at 50115-done <at> debbugs.gnu.org (full text, mbox):

From: Paul Eggert <eggert <at> cs.ucla.edu>
To: Jeremy Cantrell <jmcantrell <at> gmail.com>
Cc: Gnulib bugs <bug-gnulib <at> gnu.org>, 50115-done <at> debbugs.gnu.org
Subject: Re: bug#50115: date command arithmetic involving the epoch produces
 "invalid date"
Date: Sat, 5 Feb 2022 13:55:26 -0800

[Message part 1 (text/plain, inline)]

Thanks for the bug report. I installed the attached patches to Gnulib 
and to Coreutils, and the fix should be in the next Coreutils release.

[0001-parse-datetime-allow-calculations-to-yield-1.patch (text/x-patch, attachment)]

[0001-build-update-gnulib-submodule-to-latest.patch (text/x-patch, attachment)]

[0002-date-test-against-bug-50115.patch (text/x-patch, attachment)]

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