Timezone CET/UTC not handled correct

[pjw65]

The following code (on Ubuntu Linux 24.01 shows, that CET (UTC+1) is not correct handled, the UTC time differs by 2 hours instead of 1. CEST (Summer time) differs by 3 instead of 2 hours!
Local sunrise time seems correct,

export TZ="Europe/Zurich"; Zurich="47.3774121N 8.4541624E"; ./sunwait list rise $Zurich; ./sunwait list rise gmt $Zurich; date; date -u
07:05
05:05
Do 31 Okt 2024 07:06:18 CET
Do 31 Okt 2024 06:06:18 UTC

same with "debug" parameter added:

Debug: argv[1]: >list<
Debug: argv[2]: >debug<
Debug: argv[3]: >rise<
Debug: argv[4]: >47.3774121n<
Debug: argv[5]: >8.4541624e<
Debug: All output to use local timezone (nogmt).
Debug: Now   utcTm:  Thu Oct 31 16:20:23 2024 GMT
Debug: Now localTm:  Thu Oct 31 17:20:23 2024 CET
Debug: Now UTC bias (add to GMT to get CET) hours: 1.000000
Debug: UTC Bias (hours): 1.000000
Debug: Now: Days since 2000: 9070
Debug: Target  year set to: 124
Debug: Target   mon set to: 9
Debug: Target  mday set to: 31
Debug: Target   utcTm:  Thu Oct 31 00:00:00 2024 GMT
Debug: Target localTm:  Thu Oct 31 01:00:00 2024 CET
Debug: Target UTC bias (add to GMT to get CET) hours: 1.000000
Debug: Target: Days since 2000: 9070
Debug: Co-ordinates -  Latitude: 47.377412N
Debug: Co-ordinates - Longitude: 8.454162E
Debug: Twilight - Daylight
Debug: User specified offset (hours): 0.000000
Debug: Function selected: List
Debug: sunriset.c: Sun directly south: 11.164019 UTC, Diurnal Arc = 10.155840 hours
Debug: sunriset.c: Days since 2000: 9070
07:05
Debug: argv[1]: >list<
Debug: argv[2]: >rise<
Debug: argv[3]: >debug<
Debug: argv[4]: >gmt<
Debug: argv[5]: >47.3774121n<
Debug: argv[6]: >8.4541624e<
Debug: All output to use UTC.
Debug: Now   utcTm:  Thu Oct 31 16:20:23 2024 GMT
Debug: Now localTm:  Thu Oct 31 17:20:23 2024 CET
Debug: Now UTC bias (add to GMT to get CET) hours: 1.000000
Debug: UTC Bias (hours): 1.000000
Debug: Now: Days since 2000: 9070
Debug: Target  year set to: 124
Debug: Target   mon set to: 9
Debug: Target  mday set to: 31
Debug: Target   utcTm:  Thu Oct 31 00:00:00 2024 GMT
Debug: Target localTm:  Thu Oct 31 01:00:00 2024 CET
Debug: Target UTC bias (add to GMT to get CET) hours: 1.000000
Debug: Target: Days since 2000: 9070
Debug: Co-ordinates -  Latitude: 47.377412N
Debug: Co-ordinates - Longitude: 8.454162E
Debug: Twilight - Daylight
Debug: User specified offset (hours): 0.000000
Debug: Function selected: List
Debug: sunriset.c: Sun directly south: 11.164019 UTC, Diurnal Arc = 10.155840 hours
Debug: sunriset.c: Days since 2000: 9070
05:05

[gsauthof]

I can reproduce this with current 0.9.1 sunwait release. Also with Berlin timezone (sure, also CET/CEST).

I think the root cause is in print_a_time(), i.e. it's caused by an unconventional approach of printing a timestamp into UTC format.
First, the timestamp is converted into UTC broken-down time structure via gmtime(), to which the sunrise offset is applied, followed by mktime to normalize that broken-down time structure again. Since mktime expects local time, just another gmtime() is invoked on the timestamp returned by mktime.

I changed and simplified the function like that:

--- sunwait-0.9.1.orig/print.c  2022-09-16 18:59:34.000000000 +0200
+++ sunwait-0.9.1/print.c       2025-06-01 21:14:39.809027287 +0200
@@ -66,9 +66,9 @@ static void print_a_time
 
   // Convert current time to struct tm for UTC or local timezone
   if (pGmt_OnOff == ONOFF_ON)
-  { myUtcTime   (pMidnightTimet, &tmpTm);
-    tmpTm.tm_min += (int) (pEventHour * 60.0);
-    time_t x = mktime (&tmpTm);
+  { 
+    time_t x = *pMidnightTimet;
+    x += (int) (pEventHour * 3600);
     myUtcTime   (&x, &tmpTm);
   }
   else

This seems to fix your issue, i.e. I can't reproduce it anymore, cf:

$ for d in 2024-10-31 2025-06-01; do \
       for i in nogmt gmt; do \
            TZ='Europe/Zurich' faketime -f $d' 07:06:18' ./sunwait list rise $i 47.3774121N 8.4541624E \
; done; done
07:05
06:05
05:32
03:32