Re: bug#54764: encode-time: make DST and TIMEZONE fields of the list argument optional ones

[Eli Zaretskii <eliz@gnu.org>]

> Date: Wed, 20 Apr 2022 17:11:34 -0700
> Cc: manikulin@gmail.com, emacs-orgmode@gnu.org, 54764@debbugs.gnu.org,
>  bug-gnulib@gnu.org
> From: Paul Eggert <eggert@cs.ucla.edu>
> 
> On 4/20/22 12:30, Eli Zaretskii wrote:
> 
> > I see the time samples change in jumps of 15 msec.
> 
> Could you give the first part of the output? I would like to see what 
> the the samples are jumping from and to, and how often they jump.

That "first part", as I understand what you wanted, would be too long
and tedious to examine, as the value changes once every 5500 lines.
So I've modified the test program to print the time only when it
changes, and here's the output:

  gettime_res returned 625000 ns
  time = 1650522863.038750000
  time = 1650522863.054375000
  time = 1650522863.070000000
  time = 1650522863.085625000
  time = 1650522863.101250000
  time = 1650522863.116875000
  time = 1650522863.132500000
  time = 1650522863.148125000
  time = 1650522863.163750000
  time = 1650522863.179375000
  time = 1650522863.195000000
  time = 1650522863.210625000
  time = 1650522863.226250000
  time = 1650522863.241875000
  time = 1650522863.257500000
  time = 1650522863.273125000
  time = 1650522863.288750000
  time = 1650522863.304375000
  time = 1650522863.320000000
  time = 1650522863.335625000
  time = 1650522863.351250000
  time = 1650522863.366875000
  time = 1650522863.382500000
  time = 1650522863.398125000
  time = 1650522863.413750000

> >  Which is expected
> > on MS-Windows, given the scheduler time tick, but what does that have
> > to do with the system's time resolution?
> 
> The resolution of Elisp's (time-convert nil t) is determined by the 
> smallest nonzero gap between timestamps that are returned by C's 
> current_timespec. This is the system time resolution as far as Elisp is 
> concerned, because Elisp cannot return the current time at a finer 
> resolution than what current_timespec gives it. This resolution is not 
> necessarily the same as the time resolution of the motherboard clock, OS 
> high-res timestamp, file timestamps, etc.

Then I think I don't understand the purpose of this measurement, as
applied to Emacs Lisp.  For example, you say that this is unrelated to
file timestamps, but don't we use time values for file timestamps?
And for Windows, all this does is measure the "resolution" of the
Gnulib emulation of timespec functions on MS-Windows, it tells nothing
about the real resolution of the system time values.

More generally, if the "time resolution" determined by this procedure
is different between two systems, does it mean that two time values
that are 'equal' on one of them could be NOT 'equal' on another?  And
if so, wouldn't that mean Emacs Lisp programs will be inherently not
portable?

IOW, how do you intend to incorporate this "time resolution" into
Emacs Lisp, and what will be affected by that value?