libast/tm: DST offset and contemporaneous time zone fixes.

NEWS

@@ -2,6 +2,12 @@ This documents significant changes in the dev branch of ksh 93u+m.
 For full details, see the git log at: https://github.com/ksh93/ksh
 Uppercase BUG_* IDs are shell bug IDs as used by the Modernish shell library.
 
+2026-07-25:
+
+- For 'printf %T' and the date(1) builtin, multiple problems have been
+  fixed with the conversion of Daylight Savings Time offsets, as well as
+  contemporaneous time zones.
+
 2026-07-24:
 
 - Fix for 'read -t': the timeout was not deactivated if the read operation

src/cmd/ksh93/data/builtins.c

@@ -1230,7 +1230,7 @@ const char sh_optprint[] =
 ;
 
 const char sh_optprintf[] =
-"[-1c?\n@(#)$Id: printf (ksh 93u+m) 2024-07-31 $\n]"
+"[-1c?\n@(#)$Id: printf (ksh 93u+m) 2026-07-25 $\n]"
 "[--catalog?" SH_DICT "]"
 "[+NAME?printf - write formatted output]"
 "[+DESCRIPTION?\bprintf\b writes each \astring\a operand to "

src/cmd/ksh93/include/version.h

@@ -21,7 +21,7 @@
 #include <ast_release.h>
 #include "git.h"
 
-#define SH_RELEASE_DATE	"2026-07-24"	/* must be in this format for $((.sh.version)) */
+#define SH_RELEASE_DATE	"2026-07-25"	/* must be in this format for $((.sh.version)) */
 /*
  * This comment keeps SH_RELEASE_DATE a few lines away from SH_RELEASE_SVER to avoid
  * merge conflicts when cherry-picking dev branch commits onto a release branch.

src/cmd/ksh93/tests/attributes.sh

@@ -15,6 +15,7 @@
 #            Johnothan King <johnothanking@protonmail.com>             #
 #         hyenias <58673227+hyenias@users.noreply.github.com>          #
 #                    Sertonix <sertonix@posteo.net>                    #
+#               K. Eugene Carlson <kvngncrlsn@gmail.com>               #
 #                                                                      #
 ########################################################################
 
@@ -362,9 +363,9 @@ $SHELL -c 'builtin date' >/dev/null 2>&1 &&
 # check env var changes against a builtin that uses the env var
 
 SEC=1234252800
-ETZ=EST5EDT
+ETZ=EST5EDT,M3.2.0/2:00:00,M11.1.0/2:00:00
 EDT=03
-PTZ=PST8PDT
+PTZ=PST8PDT,M3.2.0/2:00:00,M11.1.0/2:00:00
 PDT=00
 
 CMD="date -f%H \\#$SEC"

src/cmd/ksh93/tests/printf.sh

@@ -211,14 +211,62 @@ function do_test # 1:LINENO 2:printf-STRING 3:match-string
 }
 
 # The first tests require a time zone with one or more historical changes.
-format='%Y-%m-%d %H:%M:%S'
-export TZ=Europe/Riga
-
 C='Historical changes (bad time)' # https://github.com/ksh93/ksh/issues/669
+export TZ=Europe/Riga
+format='%Y-%m-%d %H:%M:%S'
 T '#236961303'				'1977-07-05 17:35:03'
+format='%s'
+T '1977-07-05 17:35:03'		'236961303'
 
 export TZ=Europe/London
+format='%Y-%m-%d %H:%M:%S'
 T '#0'					'1970-01-01 01:00:00'
+format='%s'
+T '1970-01-01 01:00:00'		'0'
+
+C='Before and after a historical change (bad time)'
+export TZ=Pacific/Apia
+format='%Y-%m-%d %H:%M:%S'
+T '#1325239200'			'2011-12-31 00:00:00'
+T '#1325239199'			'2011-12-29 23:59:59'
+format='%s'
+T '2011-12-31 00:00:00'		'1325239200'
+T '2011-12-29 23:59:59'		'1325239199'
+
+C='Historical change from one DST zone to another (bad time)'
+export TZ=Europe/Chisinau
+format='%Y-%m-%d %H:%M:%S'
+T '#642286800'				'1990-05-10 00:00:00'
+format='%s'
+T '1990-05-10 00:00:00'		'642286800'
+
+C='Historical change from DST to standard (bad time)'
+export TZ=America/Indiana/Knox
+format='%Y-%m-%d %H:%M:%S'
+T '#688626000'				'1991-10-28 00:00:00'
+format='%s'
+T '1991-10-28 00:00:00'		'688626000'
+
+C='Historical change from standard to DST (bad time)'
+export TZ=America/Kentucky/Louisville
+format='%Y-%m-%d %H:%M:%S'
+T '#126766800'				'1974-01-07 00:00:00'
+format='%s'
+T '1974-01-07 00:00:00'		'126766800'
+
+C='Historical change from standard to standard (bad time)'
+export TZ=America/Argentina/San_Juan
+format='%Y-%m-%d %H:%M:%S'
+T '#1086148800'			'2004-06-02 00:00:00'
+format='%s'
+T '2004-06-02 00:00:00'		'1086148800'
+
+C='POSIX timezone strings without rules (bad time, musl)' #https://github.com/ksh93/ksh/issues/976
+export TZ=EST5EDT
+format='%Y-%m-%d %H:%M:%S'
+T '#1274252800'			'2010-05-19 03:06:40'
+format='%s'
+T '2010-05-19 03:06:40'		'1274252800'
 
 format='%Y-%m-%d'
 export TZ=UTC

src/lib/libast/tm/tminit.c

@@ -191,12 +191,14 @@ static void
 tmlocal(time_t now)
 {
 	Tm_zone_t*		zp;
-	int			n;
 	char*			s;
 	char*			e = NULL;
-	int			i;
-	int			m;
-	int			isdst;
+	int			utc_offset;	/* offset from Coordinated Universal Time (UTC) */
+	int			utc_offset_old;
+	int			isdst;		/* nonzero if daylight saving time */
+	int			isdst_old;
+	int			dst_offset;
+	int			month;
 	char*			t;
 	struct tm*		tp;
 	char			buf[16];
@@ -236,30 +238,42 @@ tmlocal(time_t now)
 	 */
 
 	tm_info.zone = tm_info.local = &local;
-	n = tzwest(&now, &isdst);
+	utc_offset = tzwest(&now, &isdst);
+	utc_offset_old = utc_offset;
+	isdst_old = isdst;
 
 	/*
 	 * compute local DST offset by roaming
 	 * through the last 12 months until tzwest() changes
+	 * with a system-recognized DST change
 	 */
 
-	for (i = 0; i < 12; i++)
+	for (month = 1; month <= 12; month++)
 	{
 		now -= 31 * 24 * 60 * 60;
-		if ((m = tzwest(&now, &isdst)) != n)
+		/* only break offset calculations for a DST shift */
+		if ((dst_offset = tzwest(&now, &isdst)) != utc_offset && ((!isdst && isdst_old) || (isdst && !isdst_old)))
 		{
+			dst_offset -= (utc_offset - utc_offset_old);
+			utc_offset = utc_offset_old;
 			if (!isdst)
 			{
-				isdst = n;
-				n = m;
-				m = isdst;
+				utc_offset = dst_offset;
+				dst_offset = utc_offset_old;
 			}
-			m -= n;
 			break;
 		}
+		utc_offset = dst_offset;
+		if (month == 12)
+		{
+			utc_offset = utc_offset_old;
+			dst_offset = utc_offset;
+		}
 	}
-	local.west = (short)n;
-	local.dst = (short)m;
+	dst_offset -= utc_offset;
+	isdst = isdst_old;
+	local.west = (short)utc_offset;
+	local.dst = (short)dst_offset;
 
 	/*
 	 * now get the time zone names
@@ -322,7 +336,7 @@ tmlocal(time_t now)
 		{
 			if (zp->type)
 				t = zp->type;
-			if (zp->west == n && zp->dst == m)
+			if (zp->west == utc_offset && zp->dst == dst_offset)
 			{
 				local.type = t;
 				if (!local.standard)
@@ -334,7 +348,7 @@ tmlocal(time_t now)
 					if (s < e - 1)
 					{
 						*s++ = ' ';
-						tmpoff(s, (size_t)(e - s), tm_info.format[TM_DT], m, TM_DST);
+						tmpoff(s, (size_t)(e - s), tm_info.format[TM_DT], dst_offset, TM_DST);
 					}
 					s = buf;
 				}
@@ -350,13 +364,13 @@ tmlocal(time_t now)
 			 */
 
 			e = (s = buf) + sizeof(buf);
-			s = tmpoff(s, (size_t)(e - s), tm_info.format[TM_UT], n, 0);
+			s = tmpoff(s, (size_t)(e - s), tm_info.format[TM_UT], utc_offset, 0);
 			if (!local.standard)
 				local.standard = strdup(buf);
 			if (s < e - 1)
 			{
 				*s++ = ' ';
-				tmpoff(s, (size_t)(e - s), tm_info.format[TM_UT], m, TM_DST);
+				tmpoff(s, (size_t)(e - s), tm_info.format[TM_UT], dst_offset, TM_DST);
 				if (!local.daylight)
 					local.daylight = strdup(buf);
 			}

src/lib/libast/tm/tmxdate.c

@@ -172,6 +172,10 @@ tmxdate(const char* s, char** e, Time_t now)
 	int		dir;
 	int		dst;
 	int		zone;
+	int		offset;
+	int		dst_old;
+	int		west_old;
+	int		isdst_old;
 	int		c;
 	int		f;
 	int		i;
@@ -206,6 +210,11 @@ tmxdate(const char* s, char** e, Time_t now)
 	 */
 
 	tm = tmxtm(&ts, now, NULL, 1);
+
+	dst_old = tm->tm_zone->dst;
+	west_old = tm->tm_zone->west;
+	isdst_old = tm->tm_isdst;
+
 	tm_info.date = tm->tm_zone;
 	day = -1;
 	dir = 0;
@@ -1779,6 +1788,17 @@ tmxdate(const char* s, char** e, Time_t now)
 			tm->tm_mday += j;
 		}
 	}
+
+	/*
+	 * Apply an offset to correct for historical time zone changes.
+	 */
+	tm = tmxtm(tm, tmxtime(tm, zone), tm->tm_zone, 1);
+	offset = tm->tm_zone->west - west_old;
+	if (tm->tm_isdst)
+		offset += tm->tm_zone->dst - dst_old;
+	tm->tm_min += offset;
+	tmfix(tm);
+
 	now = tmxtime(tm, zone);
 	if (tm->tm_year <= 70 && tmxsec(now) > 31536000)
 	{
@@ -1787,5 +1807,13 @@ tmxdate(const char* s, char** e, Time_t now)
 	}
 	if (e)
 		*e = last;
+
+	/*
+	 * Ensure that a proper time zone has been chosen. If the time
+	 * zones differ, this means that a historical change has occurred.
+	 */
+	if (!(dst_old == tm->tm_zone->dst && west_old == tm->tm_zone->west && isdst_old == tm->tm_isdst))
+		goto reset;
+
 	return now;
 }

src/lib/libcmd/date.c

@@ -30,7 +30,7 @@
 #include <times.h>
 
 static const char usage[] =
-"[-?\n@(#)$Id: date (AT&T Research) 2011-01-27 $\n]"
+"[-?\n@(#)$Id: date (ksh 93u+m) 2026-07-25 $\n]"
 "[--catalog?" ERROR_CATALOG "]"
 "[+NAME?date - set/list/convert dates]"
 "[+DESCRIPTION?\bdate\b sets the current date and time (with appropriate"