diff --git a/NEWS.md b/NEWS.md
index eceec9d..cc09203 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -1,3 +1,54 @@
+# Version 2026.4
+Upstream version 2026d released 2026-09-11T22:21:07+00:00
+
+## Briefly:
+
+Canada’s Northwest Territories moved to permanent -06 on 2026-08-21. Obsolescent
+settings like TZ="EST5EDT" now conform better to POSIX. Fix security,
+performance and porting bugs in zic and localtime.
+
+## Changes to future timestamps
+
+Canada’s Northwest Territories will not fall back on 2026-11-01 and will stay on
+-06 year-round, matching Alberta’s recent change. Model this with its
+traditional abbreviation CST.  Although the change to permanent -06 legally took
+place on 2026-08-21, temporarily model the change to occur on 2026-11-01 at
+02:00 for the same reason as other recent temporary hacks.  (Caution: see “NOTE
+FOR 2026b TEMPORARY HACK FOR CLDR AND CANADA” below.) This affects only
+America/Inuvik as the rest of the territory is covered by America/Edmonton, for
+which the equivalent change was released in 2026c.
+
+## Changes to past timestamps
+
+Colombia’s 1992-05-02 spring forward was at 00:00, not 24:00.
+
+Iran’s 1979-05-26 spring forward was at 00:00, not 24:00. (Thanks to N.F. Hase.)
+
+The backward-compatibility names EST5EDT, CST6CDT, MST7MDT, and PST8PDT now
+conform better to POSIX.  For example, EST5EDT now always uses the abbreviation
+"EST" for standard time (now always 5 hours behind UT) and "EDT" for daylight
+saving time, whereas it formerly had different UT offsets before standard time
+was introduced and sometimes used abbreviations like "LMT", "EWT" and "EPT", all
+contrary to POSIX.  Also, though not required by POSIX these names now use US
+federal rules rather than rules of places like New York, reverting to 2024a
+behavior.  This change affects only timestamps before 1966-10-30 at 01:00
+standard time.
+
+## Other data changes
+
+The temporary hacks used for North American timekeeping changes now work around
+a libstdc++ std::chrono bug in GCC 14.1-14.4, 15.1-15.2, and 16.1; see GCC bug
+124851.  This data change does not affect TZif files or timestamps.  The change
+does not work around the related but less serious GCC bugs 116110 and 124513.
+These GCC bugs are all fixed in GCC 16.2.
+
+## Changes to documentation
+
+URLs for release tarballs in tz-link.html have been updated to reflect their new
+canonical URLs on data.iana.org.
+
+---
+
 # Version 2026.3
 Upstream version 2026c released 2026-07-08T17:23:58+00:00
 
diff --git a/VERSION b/VERSION
index c82bdf1..45826d6 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-2026.3
\ No newline at end of file
+2026.4
\ No newline at end of file
diff --git a/news.d/2026d.md b/news.d/2026d.md
new file mode 100644
index 0000000..3afc15e
--- /dev/null
+++ b/news.d/2026d.md
@@ -0,0 +1,48 @@
+# Version 2026.4
+Upstream version 2026d released 2026-09-11T22:21:07+00:00
+
+## Briefly:
+
+Canada’s Northwest Territories moved to permanent -06 on 2026-08-21. Obsolescent
+settings like TZ="EST5EDT" now conform better to POSIX. Fix security,
+performance and porting bugs in zic and localtime.
+
+## Changes to future timestamps
+
+Canada’s Northwest Territories will not fall back on 2026-11-01 and will stay on
+-06 year-round, matching Alberta’s recent change. Model this with its
+traditional abbreviation CST.  Although the change to permanent -06 legally took
+place on 2026-08-21, temporarily model the change to occur on 2026-11-01 at
+02:00 for the same reason as other recent temporary hacks.  (Caution: see “NOTE
+FOR 2026b TEMPORARY HACK FOR CLDR AND CANADA” below.) This affects only
+America/Inuvik as the rest of the territory is covered by America/Edmonton, for
+which the equivalent change was released in 2026c.
+
+## Changes to past timestamps
+
+Colombia’s 1992-05-02 spring forward was at 00:00, not 24:00.
+
+Iran’s 1979-05-26 spring forward was at 00:00, not 24:00. (Thanks to N.F. Hase.)
+
+The backward-compatibility names EST5EDT, CST6CDT, MST7MDT, and PST8PDT now
+conform better to POSIX.  For example, EST5EDT now always uses the abbreviation
+"EST" for standard time (now always 5 hours behind UT) and "EDT" for daylight
+saving time, whereas it formerly had different UT offsets before standard time
+was introduced and sometimes used abbreviations like "LMT", "EWT" and "EPT", all
+contrary to POSIX.  Also, though not required by POSIX these names now use US
+federal rules rather than rules of places like New York, reverting to 2024a
+behavior.  This change affects only timestamps before 1966-10-30 at 01:00
+standard time.
+
+## Other data changes
+
+The temporary hacks used for North American timekeeping changes now work around
+a libstdc++ std::chrono bug in GCC 14.1-14.4, 15.1-15.2, and 16.1; see GCC bug
+124851.  This data change does not affect TZif files or timestamps.  The change
+does not work around the related but less serious GCC bugs 116110 and 124513.
+These GCC bugs are all fixed in GCC 16.2.
+
+## Changes to documentation
+
+URLs for release tarballs in tz-link.html have been updated to reflect their new
+canonical URLs on data.iana.org.
\ No newline at end of file
diff --git a/src/tzdata/__init__.py b/src/tzdata/__init__.py
index a5a1077..6a72b92 100644
--- a/src/tzdata/__init__.py
+++ b/src/tzdata/__init__.py
@@ -1,6 +1,6 @@
 # IANA versions like 2020a are not valid PEP 440 identifiers; the recommended
 # way to translate the version is to use YYYY.n where `n` is a 0-based index.
-__version__ = "2026.3"
+__version__ = "2026.4"
 
 # This exposes the original IANA version number.
-IANA_VERSION = "2026c"
+IANA_VERSION = "2026d"
diff --git a/src/tzdata/zoneinfo/America/Bogota b/src/tzdata/zoneinfo/America/Bogota
index 85b9033..8b37a3d 100644
Binary files a/src/tzdata/zoneinfo/America/Bogota and b/src/tzdata/zoneinfo/America/Bogota differ
diff --git a/src/tzdata/zoneinfo/America/Inuvik b/src/tzdata/zoneinfo/America/Inuvik
index 86639f6..20b2ad1 100644
Binary files a/src/tzdata/zoneinfo/America/Inuvik and b/src/tzdata/zoneinfo/America/Inuvik differ
diff --git a/src/tzdata/zoneinfo/Asia/Tehran b/src/tzdata/zoneinfo/Asia/Tehran
index 6fd31e0..2a46cf7 100644
Binary files a/src/tzdata/zoneinfo/Asia/Tehran and b/src/tzdata/zoneinfo/Asia/Tehran differ
diff --git a/src/tzdata/zoneinfo/CST6CDT b/src/tzdata/zoneinfo/CST6CDT
index b016880..cc2cb27 100644
Binary files a/src/tzdata/zoneinfo/CST6CDT and b/src/tzdata/zoneinfo/CST6CDT differ
diff --git a/src/tzdata/zoneinfo/EST5EDT b/src/tzdata/zoneinfo/EST5EDT
index 2b6c2ee..cc944f0 100644
Binary files a/src/tzdata/zoneinfo/EST5EDT and b/src/tzdata/zoneinfo/EST5EDT differ
diff --git a/src/tzdata/zoneinfo/Iran b/src/tzdata/zoneinfo/Iran
index 6fd31e0..2a46cf7 100644
Binary files a/src/tzdata/zoneinfo/Iran and b/src/tzdata/zoneinfo/Iran differ
diff --git a/src/tzdata/zoneinfo/MST7MDT b/src/tzdata/zoneinfo/MST7MDT
index 09e54e5..f155fa9 100644
Binary files a/src/tzdata/zoneinfo/MST7MDT and b/src/tzdata/zoneinfo/MST7MDT differ
diff --git a/src/tzdata/zoneinfo/PST8PDT b/src/tzdata/zoneinfo/PST8PDT
index aaf0778..6a27aeb 100644
Binary files a/src/tzdata/zoneinfo/PST8PDT and b/src/tzdata/zoneinfo/PST8PDT differ
diff --git a/src/tzdata/zoneinfo/tzdata.zi b/src/tzdata/zoneinfo/tzdata.zi
index 079a47f..8a40631 100644
--- a/src/tzdata/zoneinfo/tzdata.zi
+++ b/src/tzdata/zoneinfo/tzdata.zi
@@ -1,4 +1,4 @@
-# version 2026c
+# version 2026d
 # redo posix_only
 # This zic input file is in the public domain.
 R d 1916 o - Jun 14 23s 1 S
@@ -264,7 +264,7 @@ R i 1977 o - Mar 21 23 1 -
 R i 1977 o - O 20 24 0 -
 R i 1978 o - Mar 24 24 1 -
 R i 1978 o - Au 5 1 0 -
-R i 1979 o - May 26 24 1 -
+R i 1979 o - May 26 0 1 -
 R i 1979 o - S 18 24 0 -
 R i 1980 o - Mar 20 24 1 -
 R i 1980 o - S 22 24 0 -
@@ -1863,7 +1863,7 @@ R x 2019 ma - Ap Su>=2 3u 0 -
 R x 2019 2021 - S Su>=2 4u 1 -
 R x 2022 o - S Su>=9 4u 1 -
 R x 2023 ma - S Su>=2 4u 1 -
-R CO 1992 o - May 3 0 1 -
+R CO 1992 o - May 2 0 1 -
 R CO 1993 o - F 6 24 0 -
 R EC 1992 o - N 28 0 1 -
 R EC 1993 o - F 5 0 0 -
@@ -1959,6 +1959,19 @@ R U 2005 o - Mar 27 2 0 -
 R U 2005 o - O 9 2 1 -
 R U 2006 2015 - Mar Su>=8 2 0 -
 R U 2006 2014 - O Su>=1 2 1 -
+R US 1918 1919 - Mar lastSu 2 1 D
+R US 1918 1919 - O lastSu 2 0 S
+R US 1942 o - F 9 2 1 D
+R US 1945 o - Au 14 23u 1 D
+R US 1945 o - S 30 2 0 S
+R US 1967 2006 - O lastSu 2 0 S
+R US 1967 1973 - Ap lastSu 2 1 D
+R US 1974 o - Ja 6 2 1 D
+R US 1975 o - F lastSu 2 1 D
+R US 1976 1986 - Ap lastSu 2 1 D
+R US 1987 2006 - Ap Su>=1 2 1 D
+R US 2007 ma - Mar Su>=8 2 1 D
+R US 2007 ma - N Su>=1 2 0 S
 Z Africa/Abidjan -0:16:8 - LMT 1912
 0 - GMT
 Z Africa/Algiers 0:12:12 - LMT 1891 Mar 16
@@ -2051,7 +2064,7 @@ Z Africa/Windhoek 1:8:24 - LMT 1892 F 8
 2 1 SAST 1943 Mar 21 2
 2 - SAST 1990 Mar 21
 2 NA %s
-Z America/Adak 12:13:22 - LMT 1867 O 19 12:44:35
+Z America/Adak 12:13:22 - LMT 1867 O 19 0:31:13u
 -11:46:38 - LMT 1900 Au 20 12
 -11 - NST 1942
 -11 u N%sT 1946
@@ -2060,7 +2073,7 @@ Z America/Adak 12:13:22 - LMT 1867 O 19 12:44:35
 -11 u B%sT 1983 O 30 2
 -10 u AH%sT 1983 N 30
 -10 u H%sT
-Z America/Anchorage 14:0:24 - LMT 1867 O 19 14:31:37
+Z America/Anchorage 14:0:24 - LMT 1867 O 19 0:31:13u
 -9:59:36 - LMT 1900 Au 20 12
 -10 - AST 1942
 -10 u A%sT 1967 Ap
@@ -2360,7 +2373,7 @@ Z America/Detroit -5:32:11 - LMT 1905
 Z America/Edmonton -7:33:52 - LMT 1906 S
 -7 Ed M%sT 1987
 -7 C M%sT 2026 Jun 18
--7 1 MDT 2026 N 1 2
+-7 C MDT 2026 N 1 2
 -6 - CST
 Z America/Eirunepe -4:39:28 - LMT 1914
 -5 B %z 1988 S 12
@@ -2502,7 +2515,9 @@ Z America/Indiana/Winamac -5:46:25 - LMT 1883 N 18 18u
 Z America/Inuvik 0 - -00 1953
 -8 Y P%sT 1979 Ap lastSu 2
 -7 Y M%sT 1980
--7 C M%sT
+-7 C M%sT 2026 Au 21
+-7 C MDT 2026 N 1 2
+-6 - CST
 Z America/Iqaluit 0 - -00 1942 Au
 -5 Y E%sT 1999 O 31 2
 -6 C C%sT 2000 O 29 2
@@ -2512,7 +2527,7 @@ Z America/Jamaica -5:7:10 - LMT 1890
 -5 - EST 1974
 -5 u E%sT 1984
 -5 - EST
-Z America/Juneau 15:2:19 - LMT 1867 O 19 15:33:32
+Z America/Juneau 15:2:19 - LMT 1867 O 19 0:31:13u
 -8:57:41 - LMT 1900 Au 20 12
 -8 - PST 1942
 -8 u P%sT 1946
@@ -2596,7 +2611,7 @@ Z America/Merida -5:58:28 - LMT 1922 Ja 1 6u
 -6 - CST 1981 D 26 2
 -5 - EST 1982 N 2 2
 -6 m C%sT
-Z America/Metlakatla 15:13:42 - LMT 1867 O 19 15:44:55
+Z America/Metlakatla 15:13:42 - LMT 1867 O 19 0:31:13u
 -8:46:18 - LMT 1900 Au 20 12
 -8 - PST 1942
 -8 u P%sT 1946
@@ -2650,7 +2665,7 @@ Z America/New_York -4:56:2 - LMT 1883 N 18 17u
 -5 u E%sT 1946
 -5 NY E%sT 1967
 -5 u E%sT
-Z America/Nome 12:58:22 - LMT 1867 O 19 13:29:35
+Z America/Nome 12:58:22 - LMT 1867 O 19 0:31:13u
 -11:1:38 - LMT 1900 Au 20 12
 -11 - NST 1942
 -11 u N%sT 1946
@@ -2788,7 +2803,7 @@ Z America/Scoresbysund -1:27:52 - LMT 1916 Jul 28
 -2 c %z 1981 Mar 29
 -1 E %z 2024 Mar 31
 -2 E %z
-Z America/Sitka 14:58:47 - LMT 1867 O 19 15:30
+Z America/Sitka 14:58:47 - LMT 1867 O 19 0:31:13u
 -9:1:13 - LMT 1900 Au 20 12
 -8 - PST 1942
 -8 u P%sT 1946
@@ -2845,7 +2860,7 @@ Z America/Toronto -5:17:32 - LMT 1895
 Z America/Vancouver -8:12:28 - LMT 1884
 -8 Va P%sT 1987
 -8 C P%sT 2026 Mar 9
--8 1 PDT 2026 N 1 2
+-8 C PDT 2026 N 1 2
 -7 - MST
 Z America/Whitehorse -9:0:12 - LMT 1900 Au 20
 -9 Y Y%sT 1965
@@ -2856,7 +2871,7 @@ Z America/Whitehorse -9:0:12 - LMT 1900 Au 20
 Z America/Winnipeg -6:28:36 - LMT 1887 Jul 16
 -6 W C%sT 2006
 -6 C C%sT
-Z America/Yakutat 14:41:5 - LMT 1867 O 19 15:12:18
+Z America/Yakutat 14:41:5 - LMT 1867 O 19 0:31:13u
 -9:18:55 - LMT 1900 Au 20 12
 -9 - YST 1942
 -9 u Y%sT 1946
@@ -3457,6 +3472,8 @@ Z Australia/Perth 7:43:24 - LMT 1895 D
 Z Australia/Sydney 10:4:52 - LMT 1895 F
 10 AU AE%sT 1971
 10 AN AE%sT
+Z CST6CDT -6 US C%sT
+Z EST5EDT -5 US E%sT
 Z Etc/GMT 0 - GMT
 Z Etc/GMT+1 -1 - %z
 Z Etc/GMT+10 -10 - %z
@@ -3807,6 +3824,8 @@ Z Indian/Maldives 4:54 - LMT 1880
 5 - %z
 Z Indian/Mauritius 3:50 - LMT 1907
 4 MU %z
+Z MST7MDT -7 US M%sT
+Z PST8PDT -8 US P%sT
 Z Pacific/Apia 12:33:4 - LMT 1892 Jul 5
 -11:26:56 - LMT 1911
 -11:30 - %z 1950
@@ -3937,7 +3956,6 @@ L America/Noronha Brazil/DeNoronha
 L America/Sao_Paulo Brazil/East
 L America/Manaus Brazil/West
 L Europe/Brussels CET
-L America/Chicago CST6CDT
 L America/Halifax Canada/Atlantic
 L America/Winnipeg Canada/Central
 L America/Toronto Canada/Eastern
@@ -3951,7 +3969,6 @@ L Pacific/Easter Chile/EasterIsland
 L America/Havana Cuba
 L Europe/Athens EET
 L America/Panama EST
-L America/New_York EST5EDT
 L Africa/Cairo Egypt
 L Europe/Dublin Eire
 L Etc/GMT Etc/GMT+0
@@ -3977,7 +3994,6 @@ L Pacific/Kwajalein Kwajalein
 L Africa/Tripoli Libya
 L Europe/Brussels MET
 L America/Phoenix MST
-L America/Denver MST7MDT
 L America/Tijuana Mexico/BajaNorte
 L America/Mazatlan Mexico/BajaSur
 L America/Mexico_City Mexico/General
@@ -4175,6 +4191,5 @@ L Atlantic/Faroe Atlantic/Faeroe
 L Europe/Kyiv Europe/Kiev
 L Asia/Nicosia Europe/Nicosia
 L Pacific/Honolulu HST
-L America/Los_Angeles PST8PDT
 L Pacific/Guadalcanal Pacific/Ponape
 L Pacific/Port_Moresby Pacific/Truk
diff --git a/src/tzdata/zoneinfo/zone.tab b/src/tzdata/zoneinfo/zone.tab
index 69d50bd..8ff275a 100644
--- a/src/tzdata/zoneinfo/zone.tab
+++ b/src/tzdata/zoneinfo/zone.tab
@@ -123,7 +123,7 @@ CA	+5024-10439	America/Regina	CST - SK (most areas)
 CA	+5017-10750	America/Swift_Current	CST - SK (midwest)
 CA	+5333-11328	America/Edmonton	CST - AB, BC(E), NT(E), SK(W)
 CA	+690650-1050310	America/Cambridge_Bay	Mountain - NU (west)
-CA	+682059-1334300	America/Inuvik	Mountain - NT (west)
+CA	+682059-1334300	America/Inuvik	CST - NT (west)
 CA	+4916-12307	America/Vancouver	MST - BC (most areas)
 CA	+4906-11631	America/Creston	MST - BC (Creston)
 CA	+5546-12014	America/Dawson_Creek	MST - BC (Dawson Cr, Ft St John)
diff --git a/src/tzdata/zoneinfo/zone1970.tab b/src/tzdata/zoneinfo/zone1970.tab
index 635eabc..0c6a15c 100644
--- a/src/tzdata/zoneinfo/zone1970.tab
+++ b/src/tzdata/zoneinfo/zone1970.tab
@@ -114,7 +114,7 @@ CA	+5024-10439	America/Regina	CST - SK (most areas)
 CA	+5017-10750	America/Swift_Current	CST - SK (midwest)
 CA	+5333-11328	America/Edmonton	CST - AB, BC(E), NT(E), SK(W)
 CA	+690650-1050310	America/Cambridge_Bay	Mountain - NU (west)
-CA	+682059-1334300	America/Inuvik	Mountain - NT (west)
+CA	+682059-1334300	America/Inuvik	CST - NT (west)
 CA	+4916-12307	America/Vancouver	MST - BC (most areas)
 CA	+5546-12014	America/Dawson_Creek	MST - BC (Dawson Cr, Ft St John)
 CA	+5848-12242	America/Fort_Nelson	MST - BC (Ft Nelson)
diff --git a/src/tzdata/zoneinfo/zonenow.tab b/src/tzdata/zoneinfo/zonenow.tab
index 9c3a8cf..1c9b44b 100644
--- a/src/tzdata/zoneinfo/zonenow.tab
+++ b/src/tzdata/zoneinfo/zonenow.tab
@@ -65,10 +65,10 @@ XX	+4916-12307	America/Vancouver	Mountain Standard (MST) - British Columbia (mos
 XX	+332654-1120424	America/Phoenix	Mountain Standard (MST) - Arizona; western Mexico; Yukon
 #
 # -07/-06 - MST/MDT (North America DST)
-XX	+394421-1045903	America/Denver	Mountain (MST/MDT) - US; Mexico near US border; northern Canada
+XX	+394421-1045903	America/Denver	Mountain (MST/MDT) - US; Mexico near US border; western Nunavut
 #
 # -07/-06 - MST/MDT (North America DST) until 2026-11-01 02:00; then CST
-XX	+5333-11328	America/Edmonton	Central Standard (CST) - Alberta and some neighbors
+XX	+5333-11328	America/Edmonton	Central Standard (CST) - Alberta, NWT, and some neighbors
 #
 # -06
 XX	-0054-08936	Pacific/Galapagos	Galápagos
diff --git a/src/tzdata/zones b/src/tzdata/zones
index d4c3ef5..aa0f556 100644
--- a/src/tzdata/zones
+++ b/src/tzdata/zones
@@ -239,6 +239,8 @@ Australia/Lord_Howe
 Australia/Melbourne
 Australia/Perth
 Australia/Sydney
+CST6CDT
+EST5EDT
 Etc/GMT
 Etc/GMT+1
 Etc/GMT+10
@@ -309,6 +311,8 @@ Factory
 Indian/Chagos
 Indian/Maldives
 Indian/Mauritius
+MST7MDT
+PST8PDT
 Pacific/Apia
 Pacific/Auckland
 Pacific/Bougainville
@@ -355,7 +359,6 @@ Brazil/DeNoronha
 Brazil/East
 Brazil/West
 CET
-CST6CDT
 Canada/Atlantic
 Canada/Central
 Canada/Eastern
@@ -369,7 +372,6 @@ Chile/EasterIsland
 Cuba
 EET
 EST
-EST5EDT
 Egypt
 Eire
 Etc/GMT+0
@@ -395,7 +397,6 @@ Kwajalein
 Libya
 MET
 MST
-MST7MDT
 Mexico/BajaNorte
 Mexico/BajaSur
 Mexico/General
@@ -593,6 +594,5 @@ Atlantic/Faeroe
 Europe/Kiev
 Europe/Nicosia
 HST
-PST8PDT
 Pacific/Ponape
 Pacific/Truk