[tz] Irish Standard Time vs Irish Summer Time

[Paul.Koning at dell.com]

> On Jan 19, 2018, at 2:22 AM, Robert Elz <kre at munnari.OZ.AU> wrote:
> 
>  From: Tom Lane <tgl at sss.pgh.pa.us>
>  Date: Fri, 19 Jan 2018 01:07:18 -0500
>  Subject: Re: [tz] Irish Standard Time vs Irish Summer Time
> 
>  | How, exactly, will it improve anyone's life to change from "tm_isdst = 1
>  | means that local time is advanced" to "tm_isdst means something, but
>  | nobody knows what"?
> 
> The issue is that "means the local time is advanced" is, and always
> has been, an invalid assumption based uon apparent observed behaviour,
> rather than any definition.
> 
> In the past software sometimes assumed it meant "advanced by 1 hour"
> but that's been broken for so long that it is generally been forgotten
> now (I hope.)
> 
> The original use of tm_isdst was an as index into the tzname[] array
> to allow software to get the correct abbreviation to display. ...

I can believe that this is one usage that has appeared in some code at some time.

"Well defined semantics" requires having a meaning that's clearly and consistently documented and well understood to be the agreed to meaning.  It's not clear that applies to the is_dst field.  Does POSIX say anything?

A sample from a couple of manpages shows one (Mac OS, which takes it from BSD) describing it as "is summer time in effect" while Linux says "daylight savings time".  These both match "local time is advanced". 

So it seems to me that calling "local time is advanced" an "invalid assumption" is not justified.  Perhaps it would be valid to call it overly optimistic since it's based on informal definitions (manpages from certain OS implementations) but there clearly is non-zero basis for applying that definition.

	paul