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

[Paul Eggert <eggert@cs.ucla.edu>]

On 4/14/22 06:19, Max Nikulin wrote:

>  date-time + 
> "America/Los_Angeles" input should not be reduced to timezone offset in 
> the output.

It depends on the application. For some applications (e.g., generating 
"Date:" lines in email), it is entirely correct to output a timestamp 
like "14 Apr 2022 15:16:04 -0700", thus losing the fact that the 
timestamp was generated with TZ="America/Los_Angeles".

> Zone internal object or identifier is important for calculation of other date-time values based on the origin value.

Again, that depends on the application. It's typically wrong to store an 
old timestamp in a form like "1950-07-01 00:00 Europe/Lisbon", because 
there is no standard for what "Europe/Lisbon" means. If you update your 
copy of TZDB, or interpret such a timestamp on another computer, that 
can change the interpretation of such a timestamp. In this particular 
case, a change in TZDB release 2021b altered the interpretation of this 
old timestamp because we discovered that DST was observed in 1950 in 
Portugal.

If you want to keep the TZDB identifier for advice about how to 
interpret dates relative to a timestamp, that's fine. But you should 
keep the UT offset in addition to the TZDB identifier, if you want your 
app to be fully accurate and useful. For example, you should store 
"1950-07-01 00:00:00 +0000 Europe/Lisbon" for a timestamp generated by 
TZDB release 2021a, so that when you interpret the timestamp in release 
2021b you'll have an idea of what you're dealing with.

> I want hints like "in the case of ambiguity resolve to transition time immediately before/immediately after transition" or "provide suitable time prior to/after to transition".

Although that might be nice it's not what mktime gives us, and I doubt 
whether it's a good idea to try to implement it from scratch in Emacs.

> I hope, they may work without explicitly providing time zone offset to the input that anyway requires additional calculations. 

It doesn't require additional calculations on the Emacs Lisp user's 
part. All you need to do is save the UT offset, and use it later. 
There's so little overhead to this that it's not worth worrying about.

> ±n hours may mean ±n*3600 seconds or time with same minutes and seconds 
> values but hours value is changed by n even if a 30 min DST transition 
> happens in between.

Sorry, I don't understand what this sentence is intended to mean.

> `parse-time-string' has another set of problems.

Sure, but that was just an example. You can write your own date parser. 
The point is that when you save a localtime timestamp, you should save 
its UT offset too, in whatever notation is appropriate.

> UTC offset is another feature and implementing the hints I have 
> tried to describe may require implementing from scratch full stack of 
> time handling functions.

I doubt whether that's a good idea. I've written that sort of code, and 
it's a lot more work than one might think and it's notoriously difficult 
to do it correctly. You have better things to do.
> So I still do not see any point in mandatory DST and ZONE fields in new 
> interface of `encode-time'.

I think we're in agreement here. As I mentioned earlier, I plan to 
modify Emacs encode-time so that you can pass it a 6-arg list as well as 
an 9-arg list. Once this change is in, the DST and ZONE fields will not 
be mandatory.