[tz] [PROPOSED 3/3] Vanguard form now uses subsecond precision

July 28, 2022

* NEWS: Mention this.
* africa, asia, europe, northamerica, southamerica:
Add comments for Zone and continuation lines that should have
subsecond precision.
* ziguard.awk (round_to_second, get_rounding_subst):
New functions.
(BEGIN): New initializations.
(DATAFORM != "main"): Use the comments to prefer subsecond
precision in vanguard form, whole seconds otherwise.

* europe (Atlantic/Madeira): Fix typo in vanguard section.
---
 NEWS         | 10 +++++++++-
 africa       |  3 ++-
 asia         | 19 ++++++++++++-------
 europe       | 13 +++++++------
 northamerica | 18 +++++++++++-------
 southamerica | 15 ++++++++++++++-
 ziguard.awk  | 53 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 7 files changed, 108 insertions(+), 23 deletions(-)

diff --git a/NEWS b/NEWS
index 34621ed3..fdc87544 100644
--- a/NEWS
+++ b/NEWS
@@ -6,7 +6,7 @@ Unreleased, experimental changes
     Iran no longer observes DST after 2022.
     Rename Europe/Kiev to Europe/Kyiv.
     New zic -R option
-    Vanguard form now uses %z.
+    Vanguard form now uses %z and subsecond precision.
     New tailored_tarballs target, replacing rearguard_tarballs
 
   Changes to future timestamps
@@ -71,6 +71,14 @@ Unreleased, experimental changes
     to use %z eventually; in the meantime maintainers of zi parsers
     are encouraged to test the parsers on vanguard.zi.
 
+    Source data in vanguard form now uses subsecond precision,
+    introduced in release 2018d.  For example, for America/New_York
+    the UT offset before 1883-11-18 is now -4:56:01.6, and the
+    transition time that day is now 12:03:58.4.  Although TZif files
+    have only 1-second resolution so zic rounds these values to
+    -4:56:02 and 12:03:58 respectively, programs other than zic can
+    use the extra precision in the few cases where it is available.
+
     The Makefile has a new tailored_tarballs target for generating
     special-purpose tarballs.  It generalizes and replaces the
     rearguard_tarballs target and related targets and macros, which
diff --git a/africa b/africa
index b398f66d..382f5794 100644
--- a/africa
+++ b/africa
@@ -169,7 +169,7 @@ Link Africa/Abidjan Atlantic/St_Helena	# St Helena
 # Egypt
 
 # Milne says Cairo used 2:05:08.9, the local mean time of the Abbasizeh
-# observatory; round to nearest.  Milne also says that the official time for
+# observatory.  Milne also says that the official time for
 # Egypt was mean noon at the Great Pyramid, 2:04:30.5, but apparently this
 # did not apply to Cairo, Alexandria, or Port Said.
 
@@ -354,6 +354,7 @@ Rule	Egypt	2014	only	-	Jul	31	24:00	1:00	S
 Rule	Egypt	2014	only	-	Sep	lastThu	24:00	0	-
 
 # Zone	NAME		STDOFF	RULES	FORMAT	[UNTIL]
+		#STDOFF	2:05:08.9
 Zone	Africa/Cairo	2:05:09 -	LMT	1900 Oct
 			2:00	Egypt	EE%sT
 
diff --git a/asia b/asia
index ff65a048..468d2c1c 100644
--- a/asia
+++ b/asia
@@ -542,7 +542,7 @@ Rule	PRC	1987	1991	-	Apr	Sun>=11	 2:00	1:00	D
 # Zhongyuan Time ("Central plain Time") UT +08
 # Now part of Asia/Shanghai.
 # most of China
-# Milne gives 8:05:43.2 for Xujiahui Observatory time; round to nearest.
+# Milne gives 8:05:43.2 for Xujiahui Observatory time....
 # Guo says Shanghai switched to UT +08 "from the end of the 19th century".
 #
 # Long-shu Time (probably as Long and Shu were two names of the area) UT +07
@@ -661,6 +661,7 @@ Rule	PRC	1987	1991	-	Apr	Sun>=11	 2:00	1:00	D
 
 # Zone	NAME		STDOFF	RULES	FORMAT	[UNTIL]
 # Beijing time, used throughout China; represented by Shanghai.
+		#STDOFF	8:05:43.2
 Zone	Asia/Shanghai	8:05:43	-	LMT	1901
 			8:00	Shang	C%sT	1949 May 28
 			8:00	PRC	C%sT
@@ -672,7 +673,7 @@ Zone	Asia/Urumqi	5:50:20	-	LMT	1928
 
 # Hong Kong
 
-# Milne gives 7:36:41.7; round this.
+# Milne gives 7:36:41.7.
 
 # From Lee Yiu Chung (2009-10-24):
 # I found there are some mistakes for the...DST rule for Hong
@@ -856,6 +857,7 @@ Rule	HK	1973	only	-	Dec	30	3:30	1:00	S
 Rule	HK	1979	only	-	May	13	3:30	1:00	S
 Rule	HK	1979	only	-	Oct	21	3:30	0	-
 # Zone	NAME		STDOFF	RULES	FORMAT	[UNTIL]
+		#STDOFF	7:36:41.7			     0:36:41.7
 Zone	Asia/Hong_Kong	7:36:42 -	LMT	1904 Oct 30  0:36:42
 			8:00	-	HKT	1941 Jun 15  3:00
 			8:00	1:00	HKST	1941 Oct  1  4:00
@@ -1331,7 +1333,7 @@ Zone	Asia/Kolkata	5:53:28 -	LMT	1854 Jun 28 # Kolkata
 #
 # From Paul Eggert (2014-09-06):
 # The 1876 Report of the Secretary of the [US] Navy, p 306 says that Batavia
-# civil time was 7:07:12.5; round to even for Jakarta.
+# civil time was 7:07:12.5.
 #
 # From Gwillim Law (2001-05-28), overriding Shanks & Pottenger:
 # http://www.sumatera-inc.com/go_to_invest/about_indonesia.asp#standtime
@@ -1367,6 +1369,7 @@ Zone	Asia/Kolkata	5:53:28 -	LMT	1854 Jun 28 # Kolkata
 #
 # Zone	NAME		STDOFF	RULES	FORMAT	[UNTIL]
 # Java, Sumatra
+		#STDOFF	7:07:12.5
 Zone Asia/Jakarta	7:07:12 -	LMT	1867 Aug 10
 # Shanks & Pottenger say the next transition was at 1924 Jan 1 0:13,
 # but this must be a typo.
@@ -3868,7 +3871,8 @@ Zone	Asia/Samarkand	4:27:53 -	LMT	1924 May  2
 			6:00	-	+06	1982 Apr  1
 			5:00 RussiaAsia	+05/+06	1992
 			5:00	-	+05
-# Milne says Tashkent was 4:37:10.8; round to nearest.
+# Milne says Tashkent was 4:37:10.8.
+		#STDOFF	4:37:10.8
 Zone	Asia/Tashkent	4:37:11 -	LMT	1924 May  2
 			5:00	-	+05	1930 Jun 21
 			6:00 RussiaAsia	+06/+07	1991 Mar 31  2:00
@@ -3887,7 +3891,7 @@ Zone	Asia/Tashkent	4:37:11 -	LMT	1924 May  2
 # The English-language name of Vietnam's most populous city is "Ho Chi Minh
 # City"; use Ho_Chi_Minh below to avoid a name of more than 14 characters.
 
-# From Paul Eggert (2014-10-21) after a heads-up from Trần Ngọc Quân:
+# From Paul Eggert (2022-07-27) after a 2014 heads-up from Trần Ngọc Quân:
 # Trần Tiến Bình's authoritative book "Lịch Việt Nam: thế kỷ XX-XXI (1901-2100)"
 # (Nhà xuất bản Văn Hoá - Thông Tin, Hanoi, 2005), pp 49-50,
 # is quoted verbatim in:
@@ -3899,8 +3903,8 @@ Zone	Asia/Tashkent	4:37:11 -	LMT	1924 May  2
 # The 1906 transition was effective July 1 and standardized Indochina to
 # Phù Liễn Observatory, legally 104° 17' 17" east of Paris.
 # It's unclear whether this meant legal Paris Mean Time (00:09:21) or
-# the Paris Meridian (2° 20' 14.03" E); the former yields 07:06:30.1333...
-# and the latter 07:06:29.333... so either way it rounds to 07:06:30,
+# the Paris Meridian (2° 20' 14.03" E); the former yields 07:06:30.1333...,
+# the latter 07:06:29.333..., and for now guess the former,
 # which is used below even though the modern-day Phù Liễn Observatory
 # is closer to 07:06:31.  Abbreviate Phù Liễn Mean Time as PLMT.
 #
@@ -3927,6 +3931,7 @@ Zone	Asia/Tashkent	4:37:11 -	LMT	1924 May  2
 # NXB Thuận Hoá, Huế, 1995.
 
 # Zone	NAME		STDOFF	RULES	FORMAT	[UNTIL]
+		#STDOFF	7:06:30.133333333
 Zone Asia/Ho_Chi_Minh	7:06:40 -	LMT	1906 Jul  1
 			7:06:30	-	PLMT	1911 May  1 # Phù Liễn MT
 			7:00	-	+07	1942 Dec 31 23:00
diff --git a/europe b/europe
index ec7ac5c6..0bed8891 100644
--- a/europe
+++ b/europe
@@ -303,8 +303,7 @@
 # UT-00:25:22 and cites the International Telegraph Bureau.  As it is
 # not clear that there was any practical significance to the change
 # from UT-00:25:22 to UT-00:25:21.1 in civil timekeeping, omit this
-# transition for now and just use the latter value, omitting its
-# fraction since our format cannot represent fractions.
+# transition for now and just use the latter value.
 
 # "Countess Markievicz ... claimed that the [1916] abolition of Dublin Mean Time
 # was among various actions undertaken by the 'English' government that
@@ -538,6 +537,7 @@ Rule	Eire	1990	1995	-	Oct	Sun>=22	 1:00u	-1:00	-
 Rule	Eire	1996	max	-	Oct	lastSun	 1:00u	-1:00	-
 
 # Zone	NAME		STDOFF	RULES	FORMAT	[UNTIL]
+		#STDOFF	-0:25:21.1
 Zone	Europe/Dublin	-0:25:21 -	LMT	1880 Aug  2
 			-0:25:21 -	DMT	1916 May 21  2:00s
 			-0:25:21 1:00	IST	1916 Oct  1  2:00s
@@ -1298,10 +1298,10 @@ Rule	Finland	1942	only	-	Oct	4	1:00	0	-
 Rule	Finland	1981	1982	-	Mar	lastSun	2:00	1:00	S
 Rule	Finland	1981	1982	-	Sep	lastSun	3:00	0	-
 
-# Milne says Helsinki (Helsingfors) time was 1:39:49.2 (official document);
-# round to nearest.
+# Milne says Helsinki (Helsingfors) time was 1:39:49.2 (official document).
 
 # Zone	NAME		STDOFF	RULES	FORMAT	[UNTIL]
+		#STDOFF	1:39:49.2
 Zone	Europe/Helsinki	1:39:49 -	LMT	1878 May 31
 			1:39:49	-	HMT	1921 May    # Helsinki Mean Time
 			2:00	Finland	EE%sT	1983
@@ -2304,7 +2304,6 @@ Zone	Europe/Warsaw	1:24:00 -	LMT	1880
 # According to a Portuguese decree (1911-05-26)
 # https://dre.pt/application/dir/pdf1sdip/1911/05/12500/23132313.pdf
 # Lisbon was at -0:36:44.68, but switched to GMT on 1912-01-01 at 00:00.
-# Round the old offset to -0:36:45.  This agrees with Willett....
 #
 # From Michael Deckers (2018-02-15):
 # article 5 [of the 1911 decree; Deckers's translation] ...:
@@ -2391,6 +2390,7 @@ Rule	Port	1981	1982	-	Mar	lastSun	 1:00s	1:00	S
 Rule	Port	1983	only	-	Mar	lastSun	 2:00s	1:00	S
 #
 # Zone	NAME		STDOFF	RULES	FORMAT	[UNTIL]
+		#STDOFF	-0:36:44.68
 Zone	Europe/Lisbon	-0:36:45 -	LMT	1884
 			-0:36:45 -	LMT	1912 Jan  1  0:00u # Lisbon MT
 			 0:00	Port	WE%sT	1966 Apr  3  2:00
@@ -3034,11 +3034,12 @@ Zone Europe/Ulyanovsk	 3:13:36 -	LMT	1919 Jul  1  0:00u
 # Note: Effective 2005-12-01, (59) Perm Oblast and (81) Komi-Permyak
 # Autonomous Okrug merged to form (90, RU-PER) Perm Krai.
 
-# Milne says Yekaterinburg was 4:02:32.9; round to nearest.
+# Milne says Yekaterinburg was 4:02:32.9.
 # Byalokoz 1919 says its provincial time was based on Perm, at 3:45:05.
 # Assume it switched on 1916-07-03, the time of the new standard.
 # The 1919 and 1930 transitions are from Shanks.
 
+		#STDOFF	 4:02:32.9
 Zone Asia/Yekaterinburg	 4:02:33 -	LMT	1916 Jul  3
 			 3:45:05 -	PMT	1919 Jul 15  4:00
 			 4:00	-	+04	1930 Jun 21
diff --git a/northamerica b/northamerica
index fb5c94bb..a8858698 100644
--- a/northamerica
+++ b/northamerica
@@ -344,8 +344,7 @@ Zone	PST8PDT		 -8:00	US	P%sT
 # From Paul Eggert (2014-09-06):
 # Monthly Notices of the Royal Astronomical Society 44, 4 (1884-02-08), 208
 # says that New York City Hall time was 3 minutes 58.4 seconds fast of
-# Eastern time (i.e., -4:56:01.6) just before the 1883 switch.  Round to the
-# nearest second.
+# Eastern time (i.e., -4:56:01.6) just before the 1883 switch.
 
 # Rule	NAME	FROM	TO	-	IN	ON	AT	SAVE	LETTER
 Rule	NYC	1920	only	-	Mar	lastSun	2:00	1:00	D
@@ -354,6 +353,7 @@ Rule	NYC	1921	1966	-	Apr	lastSun	2:00	1:00	D
 Rule	NYC	1921	1954	-	Sep	lastSun	2:00	0	S
 Rule	NYC	1955	1966	-	Oct	lastSun	2:00	0	S
 # Zone	NAME		STDOFF	RULES	FORMAT	[UNTIL]
+		#STDOFF	-4:56:01.6			    12:03:58.4
 Zone America/New_York	-4:56:02 -	LMT	1883 Nov 18 12:03:58
 			-5:00	US	E%sT	1920
 			-5:00	NYC	E%sT	1942
@@ -2818,7 +2818,7 @@ Zone America/Tijuana	-7:48:04 -	LMT	1922 Jan  1  0:11:56
 
 # Barbados
 
-# For 1899 Milne gives -3:58:29.2; round that.
+# For 1899 Milne gives -3:58:29.2.
 
 # From P Chan (2020-12-09 and 2020-12-11):
 # Standard time of GMT-4 was adopted in 1911.
@@ -2862,6 +2862,7 @@ Rule	Barb	1978	1980	-	Apr	Sun>=15	2:00	1:00	D
 Rule	Barb	1979	only	-	Sep	30	2:00	0	S
 Rule	Barb	1980	only	-	Sep	25	2:00	0	S
 # Zone	NAME		STDOFF	RULES	FORMAT	[UNTIL]
+		#STDOFF	-3:58:29.2
 Zone America/Barbados	-3:58:29 -	LMT	1911 Aug 28 # Bridgetown
 			-4:00	Barb	A%sT	1944
 			-4:00	Barb	AST/-0330 1945
@@ -2922,10 +2923,10 @@ Zone	America/Belize	-5:52:48 -	LMT	1912 Apr  1
 
 # Bermuda
 
-# From Paul Eggert (2020-11-24):
+# From Paul Eggert (2022-07-27):
 # For 1899 Milne gives -4:19:18.3 as the meridian of the clock tower,
 # Bermuda dockyard, Ireland I.  This agrees with standard offset given in the
-# Daylight Saving Act, 1917 cited below.  Round that to the nearest second.
+# Daylight Saving Act, 1917 cited below.
 # It is not known when this time became standard for Bermuda; guess 1890.
 # The transition to -04 was specified by:
 # 1930: The Time Zone Act, 1929 (1929: No. 39) [1929-11-08]
@@ -3020,6 +3021,7 @@ Rule	Bermuda	1956	only	-	May	Sun>=22	 2:00	1:00	D
 Rule	Bermuda	1956	only	-	Oct	lastSun	 2:00	0	S
 
 # Zone	NAME		STDOFF	RULES	FORMAT	[UNTIL]
+		#STDOFF	-4:19:18.3
 Zone Atlantic/Bermuda	-4:19:18 -	LMT	1890	# Hamilton
 			-4:19:18 Bermuda BMT/BST 1930 Jan 1  2:00
 			-4:00	Bermuda	A%sT	1974 Apr 28  2:00
@@ -3034,7 +3036,7 @@ Zone Atlantic/Bermuda	-4:19:18 -	LMT	1890	# Hamilton
 
 # Costa Rica
 
-# Milne gives -5:36:13.3 as San José mean time; round to nearest.
+# Milne gives -5:36:13.3 as San José mean time.
 
 # Rule	NAME	FROM	TO	-	IN	ON	AT	SAVE	LETTER/S
 Rule	CR	1979	1980	-	Feb	lastSun	0:00	1:00	D
@@ -3046,6 +3048,7 @@ Rule	CR	1991	only	-	Jul	 1	0:00	0	S
 Rule	CR	1992	only	-	Mar	15	0:00	0	S
 # There are too many San Josés elsewhere, so we'll use 'Costa Rica'.
 # Zone	NAME		STDOFF	RULES	FORMAT	[UNTIL]
+		#STDOFF	-5:36:13.3
 Zone America/Costa_Rica	-5:36:13 -	LMT	1890        # San José
 			-5:36:13 -	SJMT	1921 Jan 15 # San José Mean Time
 			-6:00	CR	C%sT
@@ -3468,7 +3471,7 @@ Zone America/Tegucigalpa -5:48:52 -	LMT	1921 Apr
 # Jamaica
 # Shanks & Pottenger give -5:07:12, but Milne records -5:07:10.41 from an
 # unspecified official document, and says "This time is used throughout the
-# island".  Go with Milne.  Round to the nearest second as required by zic.
+# island".  Go with Milne.
 #
 # Shanks & Pottenger give April 28 for the 1974 spring-forward transition, but
 # Lance Neita writes that Prime Minister Michael Manley decreed it January 5.
@@ -3481,6 +3484,7 @@ Zone America/Tegucigalpa -5:48:52 -	LMT	1921 Apr
 # http://www.jamaicaobserver.com/columns/The-politician-in-all-of-us_17573647
 #
 # Zone	NAME		STDOFF	RULES	FORMAT	[UNTIL]
+		#STDOFF	-5:07:10.41
 Zone	America/Jamaica	-5:07:10 -	LMT	1890        # Kingston
 			-5:07:10 -	KMT	1912 Feb    # Kingston Mean Time
 			-5:00	-	EST	1974
diff --git a/southamerica b/southamerica
index 7b0b084f..55336a2a 100644
--- a/southamerica
+++ b/southamerica
@@ -400,6 +400,7 @@ Rule	Arg	2008	only	-	Oct	Sun>=15	0:00	1:00	-
 #
 # Buenos Aires (BA), Capital Federal (CF),
 Zone America/Argentina/Buenos_Aires -3:53:48 - LMT	1894 Oct 31
+		#STDOFF	-4:16:48.25
 			-4:16:48 -	CMT	1920 May    # Córdoba Mean Time
 			-4:00	-	-04	1930 Dec
 			-4:00	Arg	-04/-03	1969 Oct  5
@@ -417,6 +418,7 @@ Zone America/Argentina/Buenos_Aires -3:53:48 - LMT	1894 Oct 31
 # - Santiago del Estero switched to -4:00 on 1991-04-01,
 #   then to -3:00 on 1991-04-26.
 #
+		#STDOFF	       -4:16:48.25
 Zone America/Argentina/Cordoba -4:16:48 - LMT	1894 Oct 31
 			-4:16:48 -	CMT	1920 May
 			-4:00	-	-04	1930 Dec
@@ -429,6 +431,7 @@ Zone America/Argentina/Cordoba -4:16:48 - LMT	1894 Oct 31
 #
 # Salta (SA), La Pampa (LP), Neuquén (NQ), Rio Negro (RN)
 Zone America/Argentina/Salta -4:21:40 - LMT	1894 Oct 31
+		#STDOFF	-4:16:48.25
 			-4:16:48 -	CMT	1920 May
 			-4:00	-	-04	1930 Dec
 			-4:00	Arg	-04/-03	1969 Oct  5
@@ -441,6 +444,7 @@ Zone America/Argentina/Salta -4:21:40 - LMT	1894 Oct 31
 #
 # Tucumán (TM)
 Zone America/Argentina/Tucuman -4:20:52 - LMT	1894 Oct 31
+		#STDOFF	-4:16:48.25
 			-4:16:48 -	CMT	1920 May
 			-4:00	-	-04	1930 Dec
 			-4:00	Arg	-04/-03	1969 Oct  5
@@ -454,6 +458,7 @@ Zone America/Argentina/Tucuman -4:20:52 - LMT	1894 Oct 31
 #
 # La Rioja (LR)
 Zone America/Argentina/La_Rioja -4:27:24 - LMT	1894 Oct 31
+		#STDOFF	-4:16:48.25
 			-4:16:48 -	CMT	1920 May
 			-4:00	-	-04	1930 Dec
 			-4:00	Arg	-04/-03	1969 Oct  5
@@ -468,6 +473,7 @@ Zone America/Argentina/La_Rioja -4:27:24 - LMT	1894 Oct 31
 #
 # San Juan (SJ)
 Zone America/Argentina/San_Juan -4:34:04 - LMT	1894 Oct 31
+		#STDOFF	-4:16:48.25
 			-4:16:48 -	CMT	1920 May
 			-4:00	-	-04	1930 Dec
 			-4:00	Arg	-04/-03	1969 Oct  5
@@ -482,6 +488,7 @@ Zone America/Argentina/San_Juan -4:34:04 - LMT	1894 Oct 31
 #
 # Jujuy (JY)
 Zone America/Argentina/Jujuy -4:21:12 -	LMT	1894 Oct 31
+		#STDOFF	-4:16:48.25
 			-4:16:48 -	CMT	1920 May
 			-4:00	-	-04	1930 Dec
 			-4:00	Arg	-04/-03	1969 Oct  5
@@ -497,6 +504,7 @@ Zone America/Argentina/Jujuy -4:21:12 -	LMT	1894 Oct 31
 #
 # Catamarca (CT), Chubut (CH)
 Zone America/Argentina/Catamarca -4:23:08 - LMT	1894 Oct 31
+		#STDOFF	-4:16:48.25
 			-4:16:48 -	CMT	1920 May
 			-4:00	-	-04	1930 Dec
 			-4:00	Arg	-04/-03	1969 Oct  5
@@ -511,6 +519,7 @@ Zone America/Argentina/Catamarca -4:23:08 - LMT	1894 Oct 31
 #
 # Mendoza (MZ)
 Zone America/Argentina/Mendoza -4:35:16 - LMT	1894 Oct 31
+		#STDOFF	-4:16:48.25
 			-4:16:48 -	CMT	1920 May
 			-4:00	-	-04	1930 Dec
 			-4:00	Arg	-04/-03	1969 Oct  5
@@ -533,6 +542,7 @@ Rule	SanLuis	2008	2009	-	Mar	Sun>=8	0:00	0	-
 Rule	SanLuis	2007	2008	-	Oct	Sun>=8	0:00	1:00	-
 
 Zone America/Argentina/San_Luis -4:25:24 - LMT	1894 Oct 31
+		#STDOFF	-4:16:48.25
 			-4:16:48 -	CMT	1920 May
 			-4:00	-	-04	1930 Dec
 			-4:00	Arg	-04/-03	1969 Oct  5
@@ -551,6 +561,7 @@ Zone America/Argentina/San_Luis -4:25:24 - LMT	1894 Oct 31
 #
 # Santa Cruz (SC)
 Zone America/Argentina/Rio_Gallegos -4:36:52 - LMT	1894 Oct 31
+		#STDOFF	-4:16:48.25
 			-4:16:48 -	CMT	1920 May
 			-4:00	-	-04	1930 Dec
 			-4:00	Arg	-04/-03	1969 Oct  5
@@ -563,6 +574,7 @@ Zone America/Argentina/Rio_Gallegos -4:36:52 - LMT	1894 Oct 31
 #
 # Tierra del Fuego, Antártida e Islas del Atlántico Sur (TF)
 Zone America/Argentina/Ushuaia -4:33:12 - LMT	1894 Oct 31
+		#STDOFF	-4:16:48.25
 			-4:16:48 -	CMT	1920 May
 			-4:00	-	-04	1930 Dec
 			-4:00	Arg	-04/-03	1969 Oct  5
@@ -1390,13 +1402,14 @@ Zone Antarctica/Palmer	0	-	-00	1965
 
 # Colombia
 
-# Milne gives 4:56:16.4 for Bogotá time in 1899; round to nearest.  He writes,
+# Milne gives 4:56:16.4 for Bogotá time in 1899.  He writes,
 # "A variation of fifteen minutes in the public clocks of Bogota is not rare."
 
 # Rule	NAME	FROM	TO	-	IN	ON	AT	SAVE	LETTER/S
 Rule	CO	1992	only	-	May	 3	0:00	1:00	-
 Rule	CO	1993	only	-	Apr	 4	0:00	0	-
 # Zone	NAME		STDOFF	RULES	FORMAT	[UNTIL]
+		#STDOFF	-4:56:16.4
 Zone	America/Bogota	-4:56:16 -	LMT	1884 Mar 13
 			-4:56:16 -	BMT	1914 Nov 23 # Bogotá Mean Time
 			-5:00	CO	-05/-04
diff --git a/ziguard.awk b/ziguard.awk
index 91f68962..171b952d 100644
--- a/ziguard.awk
+++ b/ziguard.awk
@@ -50,6 +50,35 @@ function offset_abbr(offset, \
   }
 }
 
+# Round TIMESTAMP (a +-hh:mm:ss.dddd string) to the nearest second.
+function round_to_second(timestamp, \
+			 hh, mm, ss, seconds, dot_dddd, subseconds)
+{
+  dot_dddd = timestamp
+  if (!sub(/^[+-]?[0-9]+:[0-9]+:[0-9]+\./, ".", dot_dddd))
+    return timestamp
+  hh = mm = ss = timestamp
+  sub(/^[-+]?[0-9]+:[0-9]+:/, "", ss)
+  sub(/^[-+]?[0-9]+:/, "", mm)
+  sub(/^[-+]?/, "", hh)
+  seconds = 3600 * hh + 60 * mm + ss
+  subseconds = +dot_dddd
+  seconds += 0.5 < subseconds || ((subseconds == 0.5) && (seconds % 2));
+  return sprintf("%s%d:%.2d:%.2d", timestamp ~ /^-/ ? "-" : "", \
+		 seconds / 3600, seconds / 60 % 60, seconds % 60)
+}
+
+function get_rounding_subst(time, subst)
+{
+  if (DATAFORM == "vanguard") {
+    subst[0] = round_to_second(time)
+    subst[1] = time
+  } else {
+    subst[0] = time
+    subst[1] = round_to_second(time)
+  }
+}
+
 BEGIN {
   dataform_type["vanguard"] = 1
   dataform_type["main"] = 1
@@ -57,6 +86,9 @@ BEGIN {
 
   # The command line should set DATAFORM.
   if (!dataform_type[DATAFORM]) exit 1
+
+  stdoff_subst[0] = 0
+  until_subst[0] = 0
 }
 
 /^Zone/ { zone = $2 }
@@ -175,6 +207,27 @@ DATAFORM != "main" {
     }
   }
 
+  # Prefer subseconds in vanguard form, whole seconds otherwise.
+  if ($1 == "#STDOFF") {
+    get_rounding_subst($2, stdoff_subst)
+    if ($3) {
+      get_rounding_subst($3, until_subst)
+    } else {
+      until_subst[0] = 0
+    }
+  } else if (stdoff_subst[0]) {
+    stdoff_column = 2 * /^Zone/ + 1
+    stdoff_column_val = $stdoff_column
+    if (stdoff_column_val == stdoff_subst[0]) {
+      $stdoff_column = stdoff_subst[1]
+      if (until_subst[0] && $NF == until_subst[0]) {
+	$NF = until_subst[1]
+      }
+    } else if (stdoff_column_val != stdoff_subst[1]) {
+      stdoff_subst[0] = 0
+    }
+  }
+
   if (DATAFORM == "rearguard") {
 
     # In rearguard form, change the Japan rule line with "Sat>=8 25:00"
-- 
2.34.1

    