<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body style="overflow-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;">On Nov 16, 2023, at 9:29 PM, Paul Eggert via tz <tz@iana.org> wrote:<div><br><div><blockquote type="cite"><div><span style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none; float: none; display: inline !important;">Not sure what is meant by "in effect". POSIX says "The value of tm_isdst shall be positive if Daylight Savings Time is in effect, 0 if Daylight Savings Time is not in effect, and negative if the information is not available." <</span><a href="https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/time.h.html" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px;">https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/time.h.html</a><span style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none; float: none; display: inline !important;">></span><br style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;"><br style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;"><span style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none; float: none; display: inline !important;">So the intent of POSIX is that tm_isdst>0 means daylight saving time. The DST offset from UT need not be greater than the standard-time offset. POSIX (and TZDB) even allow the two offsets to be equal, though there's not much use for that.</span><br style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;"></div></blockquote></div><br></div><div>In light of the seemingly significant confusion and divergences of understanding around exactly what “Daylight Saving Time” means in various jurisdictions (e.g. Ireland vs. North America vs. ??), perhaps it's time to consider just setting tm_isdat=-1 in all zones that do not return a fixed offset for the entire year? While this would effectively mean the retirement that field, how much modern software actually makes use of it? </div><div><br></div><div>There is precedent for something like this: the deprecation of the "struct timezone *tz" argument to gettimeofday() and settimeofday(). The man page for those functions on my Linux system (CentOS 7, man-pages-3.53.-5) has this to say about that field in the NOTES section:</div><div><br></div><div>*** snip snip ***</div><div><p style="margin: 0px; font-style: normal; font-variant-caps: normal; font-stretch: normal; font-size: 11px; line-height: normal; font-family: Menlo; font-size-adjust: none; font-kerning: auto; font-variant-alternates: normal; font-variant-ligatures: normal; font-variant-numeric: normal; font-variant-east-asian: normal; font-variant-position: normal; font-feature-settings: normal; font-optical-sizing: auto; font-variation-settings: normal;"><span style="font-variant-ligatures: no-common-ligatures">On old systems, the field </span><span style="text-decoration: underline ; font-variant-ligatures: no-common-ligatures">tz_dsttime</span><span style="font-variant-ligatures: no-common-ligatures"> contains a symbolic constant (val‐</span></p>
<p style="margin: 0px; font-style: normal; font-variant-caps: normal; font-stretch: normal; font-size: 11px; line-height: normal; font-family: Menlo; font-size-adjust: none; font-kerning: auto; font-variant-alternates: normal; font-variant-ligatures: normal; font-variant-numeric: normal; font-variant-east-asian: normal; font-variant-position: normal; font-feature-settings: normal; font-optical-sizing: auto; font-variation-settings: normal;"><span style="font-variant-ligatures: no-common-ligatures">ues are given below) that indicates in which part of the year Daylight</span></p>
<p style="margin: 0px; font-style: normal; font-variant-caps: normal; font-stretch: normal; font-size: 11px; line-height: normal; font-family: Menlo; font-size-adjust: none; font-kerning: auto; font-variant-alternates: normal; font-variant-ligatures: normal; font-variant-numeric: normal; font-variant-east-asian: normal; font-variant-position: normal; font-feature-settings: normal; font-optical-sizing: auto; font-variation-settings: normal;"><span style="font-variant-ligatures: no-common-ligatures">Saving Time is in force. (Note: this value is constant throughout the</span></p>
<p style="margin: 0px; font-style: normal; font-variant-caps: normal; font-stretch: normal; font-size: 11px; line-height: normal; font-family: Menlo; font-size-adjust: none; font-kerning: auto; font-variant-alternates: normal; font-variant-ligatures: normal; font-variant-numeric: normal; font-variant-east-asian: normal; font-variant-position: normal; font-feature-settings: normal; font-optical-sizing: auto; font-variation-settings: normal;"><span style="font-variant-ligatures: no-common-ligatures">year: it does not indicate that DST is in force, it just selects an</span></p>
<p style="margin: 0px; font-style: normal; font-variant-caps: normal; font-stretch: normal; font-size: 11px; line-height: normal; font-family: Menlo; font-size-adjust: none; font-kerning: auto; font-variant-alternates: normal; font-variant-ligatures: normal; font-variant-numeric: normal; font-variant-east-asian: normal; font-variant-position: normal; font-feature-settings: normal; font-optical-sizing: auto; font-variation-settings: normal;"><span style="font-variant-ligatures: no-common-ligatures">algorithm.) The daylight saving time algorithms defined are as fol‐</span></p>
<p style="margin: 0px; font-style: normal; font-variant-caps: normal; font-stretch: normal; font-size: 11px; line-height: normal; font-family: Menlo; font-size-adjust: none; font-kerning: auto; font-variant-alternates: normal; font-variant-ligatures: normal; font-variant-numeric: normal; font-variant-east-asian: normal; font-variant-position: normal; font-feature-settings: normal; font-optical-sizing: auto; font-variation-settings: normal;"><span style="font-variant-ligatures: no-common-ligatures">lows:</span></p></div><div><span style="font-variant-ligatures: no-common-ligatures"><br></span></div><div><span style="font-variant-ligatures: no-common-ligatures">[… list of historical timezone IDs omitted …]</span></div><div><span style="font-variant-ligatures: no-common-ligatures"><br></span></div><div><span style="font-variant-ligatures: no-common-ligatures"><p style="margin: 0px; font-style: normal; font-variant-caps: normal; font-stretch: normal; font-size: 11px; line-height: normal; font-family: Menlo; font-size-adjust: none; font-kerning: auto; font-variant-alternates: normal; font-variant-ligatures: normal; font-variant-numeric: normal; font-variant-east-asian: normal; font-variant-position: normal; font-feature-settings: normal; font-optical-sizing: auto; font-variation-settings: normal;"><span style="font-variant-ligatures: no-common-ligatures">Of course it turned out that the period in which Daylight Saving Time</span></p>
<p style="margin: 0px; font-style: normal; font-variant-caps: normal; font-stretch: normal; font-size: 11px; line-height: normal; font-family: Menlo; font-size-adjust: none; font-kerning: auto; font-variant-alternates: normal; font-variant-ligatures: normal; font-variant-numeric: normal; font-variant-east-asian: normal; font-variant-position: normal; font-feature-settings: normal; font-optical-sizing: auto; font-variation-settings: normal;"><span style="font-variant-ligatures: no-common-ligatures">is in force cannot be given by a simple algorithm, one per country;</span></p>
<p style="margin: 0px; font-style: normal; font-variant-caps: normal; font-stretch: normal; font-size: 11px; line-height: normal; font-family: Menlo; font-size-adjust: none; font-kerning: auto; font-variant-alternates: normal; font-variant-ligatures: normal; font-variant-numeric: normal; font-variant-east-asian: normal; font-variant-position: normal; font-feature-settings: normal; font-optical-sizing: auto; font-variation-settings: normal;"><span style="font-variant-ligatures: no-common-ligatures">indeed, this period is determined by unpredictable political decisions.</span></p>
<p style="margin: 0px; font-style: normal; font-variant-caps: normal; font-stretch: normal; font-size: 11px; line-height: normal; font-family: Menlo; font-size-adjust: none; font-kerning: auto; font-variant-alternates: normal; font-variant-ligatures: normal; font-variant-numeric: normal; font-variant-east-asian: normal; font-variant-position: normal; font-feature-settings: normal; font-optical-sizing: auto; font-variation-settings: normal;"><span style="font-variant-ligatures: no-common-ligatures">So this method of representing timezones has been abandoned.</span></p><div><span style="font-variant-ligatures: no-common-ligatures">*** snip snip ***</span></div><div><span style="font-variant-ligatures: no-common-ligatures"><br></span></div><div><span style="font-variant-ligatures: no-common-ligatures">It strikes me that we’ve arrived at a similar place WRT the “is_dst” field: namely, that it is too simplistic a mechanism for describing actual ground truth in 2023. POSIX provides a way to tell applications unambiguously that “we don’t know”. Perhaps we should take advantage of that?</span></div><div><span style="font-variant-ligatures: no-common-ligatures"><br></span></div><div><span style="font-variant-ligatures: no-common-ligatures">Cheers!</span></div><div><span style="font-variant-ligatures: no-common-ligatures"><br></span></div><div><span style="font-variant-ligatures: no-common-ligatures"><br></span></div><div><span style="font-variant-ligatures: no-common-ligatures"><p style="margin: 0px; font-style: normal; font-variant-caps: normal; font-stretch: normal; font-size: 11px; line-height: normal; font-family: Menlo; font-size-adjust: none; font-kerning: auto; font-variant-alternates: normal; font-variant-ligatures: normal; font-variant-numeric: normal; font-variant-east-asian: normal; font-variant-position: normal; font-feature-settings: normal; font-optical-sizing: auto; font-variation-settings: normal;"><span style="font-variant-ligatures: no-common-ligatures">|---------------------------------------------------------------------|</span></p>
<p style="margin: 0px; font-style: normal; font-variant-caps: normal; font-stretch: normal; font-size: 11px; line-height: normal; font-family: Menlo; font-size-adjust: none; font-kerning: auto; font-variant-alternates: normal; font-variant-ligatures: normal; font-variant-numeric: normal; font-variant-east-asian: normal; font-variant-position: normal; font-feature-settings: normal; font-optical-sizing: auto; font-variation-settings: normal;"><span style="font-variant-ligatures: no-common-ligatures">| Frederick F. Gleason, Jr. | Chief Developer |</span></p>
<p style="margin: 0px; font-style: normal; font-variant-caps: normal; font-stretch: normal; font-size: 11px; line-height: normal; font-family: Menlo; font-size-adjust: none; font-kerning: auto; font-variant-alternates: normal; font-variant-ligatures: normal; font-variant-numeric: normal; font-variant-east-asian: normal; font-variant-position: normal; font-feature-settings: normal; font-optical-sizing: auto; font-variation-settings: normal;"><span style="font-variant-ligatures: no-common-ligatures">| | Paravel Systems |</span></p>
<p style="margin: 0px; font-style: normal; font-variant-caps: normal; font-stretch: normal; font-size: 11px; line-height: normal; font-family: Menlo; font-size-adjust: none; font-kerning: auto; font-variant-alternates: normal; font-variant-ligatures: normal; font-variant-numeric: normal; font-variant-east-asian: normal; font-variant-position: normal; font-feature-settings: normal; font-optical-sizing: auto; font-variation-settings: normal;"><span style="font-variant-ligatures: no-common-ligatures">|---------------------------------------------------------------------|</span></p>
<p style="margin: 0px; font-style: normal; font-variant-caps: normal; font-stretch: normal; font-size: 11px; line-height: normal; font-family: Menlo; font-size-adjust: none; font-kerning: auto; font-variant-alternates: normal; font-variant-ligatures: normal; font-variant-numeric: normal; font-variant-east-asian: normal; font-variant-position: normal; font-feature-settings: normal; font-optical-sizing: auto; font-variation-settings: normal;"><span style="font-variant-ligatures: no-common-ligatures">| Never take two chronometers to sea. Always take one or three, |</span></p>
<p style="margin: 0px; font-style: normal; font-variant-caps: normal; font-stretch: normal; font-size: 11px; line-height: normal; font-family: Menlo; font-size-adjust: none; font-kerning: auto; font-variant-alternates: normal; font-variant-ligatures: normal; font-variant-numeric: normal; font-variant-east-asian: normal; font-variant-position: normal; font-feature-settings: normal; font-optical-sizing: auto; font-variation-settings: normal;"><span style="font-variant-ligatures: no-common-ligatures">| otherwise you'll never be sure what time it is. |</span></p>
<p style="margin: 0px; font-style: normal; font-variant-caps: normal; font-stretch: normal; font-size: 11px; line-height: normal; font-family: Menlo; font-size-adjust: none; font-kerning: auto; font-variant-alternates: normal; font-variant-ligatures: normal; font-variant-numeric: normal; font-variant-east-asian: normal; font-variant-position: normal; font-feature-settings: normal; font-optical-sizing: auto; font-variation-settings: normal;"><span style="font-variant-ligatures: no-common-ligatures">| |</span></p>
<p style="margin: 0px; font-style: normal; font-variant-caps: normal; font-stretch: normal; font-size: 11px; line-height: normal; font-family: Menlo; font-size-adjust: none; font-kerning: auto; font-variant-alternates: normal; font-variant-ligatures: normal; font-variant-numeric: normal; font-variant-east-asian: normal; font-variant-position: normal; font-feature-settings: normal; font-optical-sizing: auto; font-variation-settings: normal;"><span style="font-variant-ligatures: no-common-ligatures">| -- Anonymous |</span></p>
<p style="margin: 0px; font-style: normal; font-variant-caps: normal; font-stretch: normal; font-size: 11px; line-height: normal; font-family: Menlo; font-size-adjust: none; font-kerning: auto; font-variant-alternates: normal; font-variant-ligatures: normal; font-variant-numeric: normal; font-variant-east-asian: normal; font-variant-position: normal; font-feature-settings: normal; font-optical-sizing: auto; font-variation-settings: normal;"><span style="font-variant-ligatures: no-common-ligatures">|---------------------------------------------------------------------|</span></p><div><span style="font-variant-ligatures: no-common-ligatures"><br></span></div></span></div></span></div></body></html>