[tz] [PROPOSED] Treat 1931 changes in Mexico as DST

[Paul Eggert]

Mexico’s Chamber of Deputies was supposed to vote today on
legislation abolishing DST in most of Mexico.  Although the vote
was delayed due to Internet problems, news coverage of the
non-event led me to review our sources for DST in Mexico, and I
found that tzdb considered the transitions in 1931 to be two
changes to standard time, whereas it is considered to be DST in
today’s news coverage, as well as in our main source, the history
of Mexican local time published by the Investigation and Analysis
Service of the Mexican Library of Congress.  Fix this by
considering the 1931 changes to be DST, not standard time.
* NEWS: Mention this.
* northamerica (Mexico): Add DST rules for 1931.
(America/Mexico_City, America/Ojinaga, America/Chihuahua)
(America/Hermosillo, America/Mazatlan, America/Bahia_Banderas):
Use them.
---
 NEWS         |  9 +++++++++
 northamerica | 26 ++++++++------------------
 2 files changed, 17 insertions(+), 18 deletions(-)

diff --git a/NEWS b/NEWS
index 64327d0..bcdad41 100644
--- a/NEWS
+++ b/NEWS
@@ -1,5 +1,14 @@
 News for the tz database
 
+Unreleased, experimental changes
+
+  Changes to past time zone abbreviations and DST flags
+
+    The temporary advancement of clocks in central Mexico in summer
+    1931 is now treated as daylight saving time, instead of as two
+    changes to standard time.
+
+
 Release 2022d - 2022-09-23 12:02:57 -0700
 
   Briefly:
diff --git a/northamerica b/northamerica
index 13fb064..f848228 100644
--- a/northamerica
+++ b/northamerica
@@ -2617,6 +2617,8 @@ Zone America/Dawson	-9:17:40 -	LMT	1900 Aug 20
 #    longitude they are located at.
 
 # Rule	NAME	FROM	TO	-	IN	ON	AT	SAVE	LETTER/S
+Rule	Mexico	1931	only	-	May	1	23:00	1:00	D
+Rule	Mexico	1931	only	-	Oct	1	0:00	0	S
 Rule	Mexico	1939	only	-	Feb	5	0:00	1:00	D
 Rule	Mexico	1939	only	-	Jun	25	0:00	0	S
 Rule	Mexico	1940	only	-	Dec	9	0:00	1:00	D
@@ -2667,9 +2669,7 @@ Zone America/Monterrey	-6:41:16 -	LMT	1921 Dec 31 23:18:44
 Zone America/Mexico_City -6:36:36 -	LMT	1922 Jan  1  0:23:24
 			-7:00	-	MST	1927 Jun 10 23:00
 			-6:00	-	CST	1930 Nov 15
-			-7:00	-	MST	1931 May  1 23:00
-			-6:00	-	CST	1931 Oct
-			-7:00	-	MST	1932 Apr  1
+			-7:00	Mexico	M%sT	1932 Apr  1
 			-6:00	Mexico	C%sT	2001 Sep 30  2:00
 			-6:00	-	CST	2002 Feb 20
 			-6:00	Mexico	C%sT
@@ -2680,9 +2680,7 @@ Zone America/Mexico_City -6:36:36 -	LMT	1922 Jan  1  0:23:24
 Zone America/Ojinaga	-6:57:40 -	LMT	1922 Jan  1  0:02:20
 			-7:00	-	MST	1927 Jun 10 23:00
 			-6:00	-	CST	1930 Nov 15
-			-7:00	-	MST	1931 May  1 23:00
-			-6:00	-	CST	1931 Oct
-			-7:00	-	MST	1932 Apr  1
+			-7:00	Mexico	M%sT	1932 Apr  1
 			-6:00	-	CST	1996
 			-6:00	Mexico	C%sT	1998
 			-6:00	-	CST	1998 Apr Sun>=1  3:00
@@ -2692,9 +2690,7 @@ Zone America/Ojinaga	-6:57:40 -	LMT	1922 Jan  1  0:02:20
 Zone America/Chihuahua	-7:04:20 -	LMT	1921 Dec 31 23:55:40
 			-7:00	-	MST	1927 Jun 10 23:00
 			-6:00	-	CST	1930 Nov 15
-			-7:00	-	MST	1931 May  1 23:00
-			-6:00	-	CST	1931 Oct
-			-7:00	-	MST	1932 Apr  1
+			-7:00	Mexico	M%sT	1932 Apr  1
 			-6:00	-	CST	1996
 			-6:00	Mexico	C%sT	1998
 			-6:00	-	CST	1998 Apr Sun>=1  3:00
@@ -2703,9 +2699,7 @@ Zone America/Chihuahua	-7:04:20 -	LMT	1921 Dec 31 23:55:40
 Zone America/Hermosillo	-7:23:52 -	LMT	1921 Dec 31 23:36:08
 			-7:00	-	MST	1927 Jun 10 23:00
 			-6:00	-	CST	1930 Nov 15
-			-7:00	-	MST	1931 May  1 23:00
-			-6:00	-	CST	1931 Oct
-			-7:00	-	MST	1932 Apr  1
+			-7:00	Mexico	M%sT	1932 Apr  1
 			-6:00	-	CST	1942 Apr 24
 			-7:00	-	MST	1949 Jan 14
 			-8:00	-	PST	1970
@@ -2743,9 +2737,7 @@ Zone America/Hermosillo	-7:23:52 -	LMT	1921 Dec 31 23:36:08
 Zone America/Mazatlan	-7:05:40 -	LMT	1921 Dec 31 23:54:20
 			-7:00	-	MST	1927 Jun 10 23:00
 			-6:00	-	CST	1930 Nov 15
-			-7:00	-	MST	1931 May  1 23:00
-			-6:00	-	CST	1931 Oct
-			-7:00	-	MST	1932 Apr  1
+			-7:00	Mexico	M%sT	1932 Apr  1
 			-6:00	-	CST	1942 Apr 24
 			-7:00	-	MST	1949 Jan 14
 			-8:00	-	PST	1970
@@ -2755,9 +2747,7 @@ Zone America/Mazatlan	-7:05:40 -	LMT	1921 Dec 31 23:54:20
 Zone America/Bahia_Banderas	-7:01:00 -	LMT	1921 Dec 31 23:59:00
 			-7:00	-	MST	1927 Jun 10 23:00
 			-6:00	-	CST	1930 Nov 15
-			-7:00	-	MST	1931 May  1 23:00
-			-6:00	-	CST	1931 Oct
-			-7:00	-	MST	1932 Apr  1
+			-7:00	Mexico	M%sT	1932 Apr  1
 			-6:00	-	CST	1942 Apr 24
 			-7:00	-	MST	1949 Jan 14
 			-8:00	-	PST	1970
-- 
2.37.3