bug#26101: Counterproductive calculation order in date

[Ulf Zibis]

Eric,

much thanks for your detailed examination.


Am 15.03.2017 um 02:21 schrieb Eric Blake:
> Let's try this with the new --debug option of 8.26
Great, but current version of my Ubuntu is 8.25

> $ date --debug -d "$(($(date -r ChangeLog +%s)-$(date +%s))) seconds -2
> months"
> date: parsed relative part: -5363514 seconds
> date: parsed relative part: -2 month(s) -5363514 seconds
Why not:

date: parsed relative part: -5363514 seconds -2 month(s)


A more simple example without touch:
$ date +%F
2017-03-15
$ date -d "-20 day" +%F
2017-02-23
$ date -d "-20 day -2 month" +%F
2016-12-26
$ date -d "-2 month -20 day" +%F
2016-12-26

In the 2nd example I would expect:
2016-12-23

> What calculation are you trying to achieve? The inner $(()) computes the
> difference between two dates: the reference file timestamp, and the
> current time, resulting in a negative offset in seconds.  The outer date
> computation (where I ran --debug) then sees only two relative values - a
> negative seconds offset, and a negative months offset, both applied from
> the current time as the starting point.
>
> > Otherwise it seems impossible to correctly change the time stamp of a
> > file relative to its current value.
> Wait - so you're trying to shift a file's time stamp by a fixed amount,
> such as by 2 months?  Doesn't that imply that the outer date should see
> the file's timestamp as one of its starting points, rather than being
> left to start from the current time as its starting point?
Yes, but I failed with (a) combining -r and -d and (b) setting an 
absolute starting point e.g.:
$ date -d "$(date -r test) -2 month"
date: ungültiges Datum »Di 10. Jan 22:39:00 CET 2017 -2 month“

So I did this "trick", because I didn't see a chance to directly allow a 
reference file timestamp as starting point.

> So what's wrong with:
> $ date --debug -d "Jan 1 1970 $(date -r ChangeLog +%s) seconds -2 months"
> date: parsed date part: (Y-M-D) 2017-01-01
> date: parsed number part: year: 1970
> date: parsed relative part: +1484176262 seconds
> date: parsed relative part: -2 month(s) +1484176262 seconds
> date: input timezone: -06:00 (set from system default)
> date: warning: using midnight as starting time: 00:00:00
> date: starting date/time: '(Y-M-D) 1970-01-01 00:00:00 TZ=-06:00'
> date: warning: when adding relative months/years, it is recommended to
> specify the 15th of the months
> date: after date adjustment (+0 years, -2 months, +0 days),
> date:     new date/time = '(Y-M-D) 1969-11-01 00:00:00 TZ=-06:00'
> date: '(Y-M-D) 1969-11-01 00:00:00 TZ=-06:00' = -5248800 epoch-seconds
> date: after time adjustment (+0 hours, +0 minutes, +1484176262 seconds,
> +0 ns),
> date:     new time = 1478927462 epoch-seconds
> date: output timezone: -06:00 (set from system default)
> date: final: 1478927462.000000000 (epoch-seconds)
> date: final: (Y-M-D) 2016-11-12 05:11:02 (UTC0)
> date: final: (Y-M-D) 2016-11-11 23:11:02 (output timezone TZ=-06:00)
> Fri Nov 11 23:11:02 CST 2016
>
> which is two months prior to the timestamp?

This example may work, because the shortness of february is not involved.

> Also, since you're using GNU date, you're probably using GNU touch,
> which means you can use touch -d instead of touch -t to set the time of
> the file, with a LOT less notational overhead, and in a reversible
> manner relative to a timestamp:
>
> $ : > foo
> $ touch -d '-2 months' -r foo foo
> $ ls -l foo
> -rw-rw-r--. 1 eblake eblake 0 Jan 14 19:19 foo
> $ touch -d '+2 months' -r foo foo
> $ ls -l foo
> -rw-rw-r--. 1 eblake eblake 0 Mar 14 20:19 foo
Wow, much thanks, I didn't know that touch (a) would allow relative 
dates and (b) would allow to combine -r and -d and even in the weird 
order of -d -r.

> I'm leaving this open,
Thanks.

So I see several points for enhancement:
- Allow @seconds in -d option, even for touch.
- Document the '@' syntax not only as example.
- For date document if combination of -r -d (and others) is allowed.
- For touch document if combination of -r -d (and others) is allowed.
- For date allow combination of -r -d (maybe even more).
- Calculate terms of -d option from left to right.

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