In the example shown below, copied from from tzdata2018i Africa file, the last Zone line, referencing the Namibia rule, should start at 1990 March 21 2:00 am. There are no Namibia transitions defined from that start, until 1994. However zic does create a transition using the second Namibia rule at the start time, as shown in the zdump output below. There are many other cases like this, where an UNTIL specification ends well before the next applicable rule of the following zone line. I am trying to read the zic manual page as a specification, and I do not understand why zic inserts that transition. It seems to me that either the last Zone line (SAST) would remain in effect until 1994, as there is no other transition defined, or that period is in an undefined state, as the SAST transition is not in effect after1990 March 21. Can someone correct my understanding? ------- Specification extract # RULE NAME FROM TO TYPE IN ON AT SAVE LETTER/S # Vanguard section, for zic and other parsers that support negative DST. Rule Namibia 1994 only - Mar 21 0:00 -1:00 WAT Rule Namibia 1994 2017 - Sep Sun>=1 2:00 0 CAT Rule Namibia 1995 2017 - Apr Sun>=1 2:00 -1:00 WAT # Zone NAME GMTOFF RULES FORMAT [UNTIL] Zone Africa/Windhoek 1:08:24 - LMT 1892 Feb 8 1:30 - +0130 1903 Mar 2:00 - SAST 1942 Sep 20 2:00 2:00 1:00 SAST 1943 Mar 21 2:00 2:00 - SAST 1990 Mar 21 # independence # Vanguard section, for zic and other parsers that support negative DST. 2:00 Namibia %s ------ Zicdump output: $ zdump -v -c 1892,1996 Africa/Windhoek Africa/Windhoek -9223372036854775808 = NULL Africa/Windhoek -9223372036854689408 = NULL Africa/Windhoek Sun Feb 7 22:51:35 1892 UT = Sun Feb 7 23:59:59 1892 LMT isdst=0 gmtoff=4104 Africa/Windhoek Sun Feb 7 22:51:36 1892 UT = Mon Feb 8 00:21:36 1892 +0130 isdst=0 gmtoff=5400 Africa/Windhoek Sat Feb 28 22:29:59 1903 UT = Sat Feb 28 23:59:59 1903 +0130 isdst=0 gmtoff=5400 Africa/Windhoek Sat Feb 28 22:30:00 1903 UT = Sun Mar 1 00:30:00 1903 SAST isdst=0 gmtoff=7200 Africa/Windhoek Sat Sep 19 23:59:59 1942 UT = Sun Sep 20 01:59:59 1942 SAST isdst=0 gmtoff=7200 Africa/Windhoek Sun Sep 20 00:00:00 1942 UT = Sun Sep 20 03:00:00 1942 SAST isdst=1 gmtoff=10800 Africa/Windhoek Sat Mar 20 22:59:59 1943 UT = Sun Mar 21 01:59:59 1943 SAST isdst=1 gmtoff=10800 Africa/Windhoek Sat Mar 20 23:00:00 1943 UT = Sun Mar 21 01:00:00 1943 SAST isdst=0 gmtoff=7200 Africa/Windhoek Tue Mar 20 21:59:59 1990 UT = Tue Mar 20 23:59:59 1990 SAST isdst=0 gmtoff=7200 Africa/Windhoek Tue Mar 20 22:00:00 1990 UT = Wed Mar 21 00:00:00 1990 CAT isdst=0 gmtoff=7200 <------ zic inserted the 1994 "CAT" transition. How was this chosen? Africa/Windhoek Sun Mar 20 21:59:59 1994 UT = Sun Mar 20 23:59:59 1994 CAT isdst=0 gmtoff=7200 Africa/Windhoek Sun Mar 20 22:00:00 1994 UT = Sun Mar 20 23:00:00 1994 WAT isdst=1 gmtoff=3600 <----- first applicable rule in 1994. Africa/Windhoek Sun Sep 4 00:59:59 1994 UT = Sun Sep 4 01:59:59 1994 WAT isdst=1 gmtoff=3600 Africa/Windhoek Sun Sep 4 01:00:00 1994 UT = Sun Sep 4 03:00:00 1994 CAT isdst=0 gmtoff=7200 Africa/Windhoek Sat Apr 1 23:59:59 1995 UT = Sun Apr 2 01:59:59 1995 CAT isdst=0 gmtoff=7200 Africa/Windhoek Sun Apr 2 00:00:00 1995 UT = Sun Apr 2 01:00:00 1995 WAT isdst=1 gmtoff=3600 Africa/Windhoek Sun Sep 3 00:59:59 1995 UT = Sun Sep 3 01:59:59 1995 WAT isdst=1 gmtoff=3600 Africa/Windhoek Sun Sep 3 01:00:00 1995 UT = Sun Sep 3 03:00:00 1995 CAT isdst=0 gmtoff=7200 Africa/Windhoek 9223372036854689407 = NULL Africa/Windhoek 9223372036854775807 = NULL -------