bug#33221: date command + 1 month

[Assaf Gordon]

tags 33221 notabug
close 33221
stop

Hello,

On 2018-10-31 5:08 p.m., Nicolás Flores wrote:
> Date Bug detected: Wed Oct 31 17:02:41 CST 2018
> when executing
> # *date +%b*
> Result:
> *Oct *
> This is Ok, but...
>
> # *date +%b -d '+1 month'*
> Result: *Dec*
> Jump in the month of *November*

Indeed - that is the case, but it is not a bug.
It is a limitation of date's internal working .

date has a new --debug option that helps in troubleshooting
such cases:

==
  $ date --debug -d '+1 month'
  date: parsed relative part: +1 month(s)
  date: input timezone: system default
  date: using current time as starting value: '20:17:28'
  date: using current date as starting value: '(Y-M-D) 2018-10-31'
  date: starting date/time: '(Y-M-D) 2018-10-31 20:17:28'
**date: warning: when adding relative months/years, it is recommended to 
  specify the 15th of the months**
  date: after date adjustment (+0 years, +1 months, +0 days),
  date:     new date/time = '(Y-M-D) 2018-12-01 19:17:28'
  date: warning: daylight saving time changed after date adjustment
**date: warning: month/year adjustment resulted in shifted dates:**
**date:      adjusted Y M D: 2018 11 31**
  date:    normalized Y M D: 2018 12 01
  date: '(Y-M-D) 2018-12-01 19:17:28' = 1543717048 epoch-seconds
  date: timezone: system default
  date: final: 1543717048.174426822 (epoch-seconds)
  date: final: (Y-M-D) 2018-12-02 02:17:28 (UTC)
  date: final: (Y-M-D) 2018-12-01 19:17:28 (UTC-07)
  Sat Dec  1 19:17:28 MST 2018
===

I highlighted three lines in the debug output.
When you added a month, internally date converted it to:
  year: 2018
  month: 10+1 = 11
  day: 31 (today's date).

Then, date recognized that 31st of November is not a valid date,
and normalized it to Dec 1st.

The debug output includes a warning/recommendation:
when adding/subtracting months, it is recommended to start from the
15th of the month.
Example:

  $ date -d '2018-10-15 +1 month'
  Thu Nov 15 00:00:00 MST 2018
  $ date -d '2018-10-15 +1 month' +%b
  Nov

To do it in a script (obtaining the current year/month):

  $ date -d "$(date +%Y-%m-15) +1 month" +%b
  Nov

To see more similar examples (e.g. with leap years, and February), read
the following thread:
   https://lists.gnu.org/archive/html/bug-coreutils/2018-02/msg00005.html


As such, I'm closing this bug.
Discussion can continue by replying to this thread.

-assaf