[tz] Extra transition type is generated by zic with -r specified

Almaz Mingaleev mingaleev at google.com
Mon Oct 25 11:18:10 UTC 2021

I’ve tried to generate a TZif file with transitions up to 1 Jan 2100 using
zic with -r and “-b fat”. However, an extra transition type is generated
for many (all?) time zones. That type’s UTC offset is zero and it is used
only for the last transition in the 64-bit section.

What I did:
mkdir /tmp/zic
cd /tmp/zic
git clone https://github.com/eggert/tz.git .
make -C . zic
make -C . NDATA= rearguard.zi
mkdir data
./zic -b fat -r /@4102444800 -d ./data rearguard.zi

Below is an investigation for Australia/Lord_Howe (I don’t think choice
matters here). I couldn’t find a way to force zdump to print transitions
for a given TZif file. Instead, please see human-readable dumps from the
TZif from our internal tools immediately below and raw byte information at
the bottom if you’d prefer to work with that.

These are local time transition types for Australia/Lord_Howe:
gmtOffset (seconds),isDst,ttisgmt,ttisstd,[gmtOffset ISO],[DST?]
0,0,,,PT0S,STD                          <- this is odd

transition,type,[UTC time],[Type offset],[Type isDST]
4102444800,1,2100-01-01T00:00:00Z,PT0S,STD     <- this is odd

So, after the upper bound specified in -r, the transitions say that GMT is

As I’d expect, the 32-bit transitions are not affected when the upper bound
in -r is after 2038. However, with -r /@1634833073 the 32-bit section also
has 6 transition types and the last transition sets offset to 0.

Is that a bug or is there a reason for such behaviour?


Byte manipulations:

typecnt is 5 in 32-bit section: xxd -seek 36(4+1+15+4*4) -len 4
64-bit section starts at offset 674.
timecnt is 0xF0(=240) in 64-bit section: xxd -seek 706(674 + 20 + 3 * 4)
-len 4 data/Australia/Lord_Howe
typecnt is 6 in 64-bit section: xxd -seek 710(706 + 4) -len 4
So, 64-bit header has one extra transition type.

Let’s check how it is used and what that transition is:
2638 = 674 + 44 + 240 * 8
xxd -seek 2638 -len 240 -c 1 data/Australia/Lord_Howe prints transition
00000b38: 05  .
00000b39: 04  .
00000b3a: 05  .
00000b3b: 04  .
00000b3c: 05  .
00000b3d: 01  .

Transition type is changing from 4 to 5, but the last one is 1.
xxd -seek 2878 -len 36 -c 6 data/Australia/Lord_Howe prints local time type
00000b3e: 0000 9524 0004  ...$..
00000b44: 0000 0000 0000  ......   <- the last transition type
00000b4a: 0000 8ca0 0008  ......
00000b50: 0000 a1b8 010d  ......
00000b56: 0000 93a8 0013  ......
00000b5c: 0000 9ab0 0119  ......

Which means that 1st type’s offset is 0 and that the last rule for Lord
Howe is actually GMT.
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