Re: DST change issue for timezones in southern hemisphere

[Rich Felker <dalias@libc.org>]

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On Mon, Mar 06, 2017 at 12:44:53PM +0200, JS wrote:
> Hi!
> 
> For some reason DST change in southern hemisphere occurs 1 hour late
> when clock is turned backward and 1 hour early when clock is turned
> forward.
> 
> For example, timezone for spec for Brazil is:
> BRT3BRST,M10.3.0/0,M2.3.0/0
> Which means that DST change occurs at midnight.
> 
> Output of date for both DST changes:
> 
> ~# date
> Sun Feb 19 00:59:59 BRST 2017
> ~# date
> Sun Feb 19 00:00:00 BRT 2017
> 
> ~# date
> Sat Oct 14 22:59:59 BRT 2017
> ~# date
> Sun Oct 15 00:00:00 BRST 2017
> 
> I've tested the same code on same hardware but with uClibc library
> instead of musl and it works fine.

I think this is indeed a bug. Can you confirm that the correct
progressions across DST change should be:

Sat Feb 18 23:59:59 BRST 2017
Sat Feb 18 23:00:00 BRT 2017

Sat Oct 14 23:59:59 BRT 2017
Sun Oct 15 01:00:00 BRST 2017

If so I think I have a working fix (see attached). The logic for
handling DST transition times for southern hemisphere was trying to do
something special that was actually wrong and didn't need to be
special-cased.

Rich

[-- Attachment #2: southern-hemisphere.diff --]
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diff --git a/src/time/__tz.c b/src/time/__tz.c
index 0e0c4ea..ffe8d40 100644
--- a/src/time/__tz.c
+++ b/src/time/__tz.c
@@ -373,18 +373,14 @@ void __secs_to_zone(long long t, int local, int *isdst, long *offset, long *oppo
 	long long t0 = rule_to_secs(r0, y);
 	long long t1 = rule_to_secs(r1, y);
 
+	if (!local) {
+		t0 += __timezone;
+		t1 += dst_off;
+	}
 	if (t0 < t1) {
-		if (!local) {
-			t0 += __timezone;
-			t1 += dst_off;
-		}
 		if (t >= t0 && t < t1) goto dst;
 		goto std;
 	} else {
-		if (!local) {
-			t1 += __timezone;
-			t0 += dst_off;
-		}
 		if (t >= t1 && t < t0) goto std;
 		goto dst;
 	}