Add more gmtime() tests

tests/expected/time/gmtime.stdout

+Unix epoch:
+  tm_sec    = 0
+  tm_min    = 0
+  tm_hour   = 0
+  tm_mday   = 1
+  tm_mon    = 0
+  tm_year   = 70
+  tm_wday   = 4
+  tm_yday   = 0
+  tm_isdst  = 0
+  tm_gmtoff = 0
+  tm_zone   = UTC
+
+1970-03-01 00:00:00:
+  tm_sec    = 0
+  tm_min    = 0
+  tm_hour   = 0
+  tm_mday   = 1
+  tm_mon    = 2
+  tm_year   = 70
+  tm_wday   = 0
+  tm_yday   = 59
+  tm_isdst  = 0
+  tm_gmtoff = 0
+  tm_zone   = UTC
+
+1970-12-31 23:59:59:
+  tm_sec    = 59
+  tm_min    = 59
+  tm_hour   = 23
+  tm_mday   = 31
+  tm_mon    = 11
+  tm_year   = 70
+  tm_wday   = 4
+  tm_yday   = 364
+  tm_isdst  = 0
+  tm_gmtoff = 0
+  tm_zone   = UTC
+
+1972-02-29 23:59:59:
+  tm_sec    = 59
+  tm_min    = 59
+  tm_hour   = 23
+  tm_mday   = 29
+  tm_mon    = 1
+  tm_year   = 72
+  tm_wday   = 2
+  tm_yday   = 59
+  tm_isdst  = 0
+  tm_gmtoff = 0
+  tm_zone   = UTC
+
+1972-12-31 23:59:59:
+  tm_sec    = 59
+  tm_min    = 59
+  tm_hour   = 23
+  tm_mday   = 31
+  tm_mon    = 11
+  tm_year   = 72
+  tm_wday   = 0
+  tm_yday   = 365
+  tm_isdst  = 0
+  tm_gmtoff = 0
+  tm_zone   = UTC
+
+2000-02-29 23:59:59:
+  tm_sec    = 59
+  tm_min    = 59
+  tm_hour   = 23
+  tm_mday   = 29
+  tm_mon    = 1
+  tm_year   = 100
+  tm_wday   = 2
+  tm_yday   = 59
+  tm_isdst  = 0
+  tm_gmtoff = 0
+  tm_zone   = UTC
+
+2000-12-31 23:59:59:
+  tm_sec    = 59
+  tm_min    = 59
+  tm_hour   = 23
+  tm_mday   = 31
+  tm_mon    = 11
+  tm_year   = 100
+  tm_wday   = 0
+  tm_yday   = 365
+  tm_isdst  = 0
+  tm_gmtoff = 0
+  tm_zone   = UTC
+
+2100-03-01 00:00:00:
+  tm_sec    = 0
+  tm_min    = 0
+  tm_hour   = 0
+  tm_mday   = 1
+  tm_mon    = 2
+  tm_year   = 200
+  tm_wday   = 1
+  tm_yday   = 59
+  tm_isdst  = 0
+  tm_gmtoff = 0
+  tm_zone   = UTC
+
+2100-12-31 23:59:59:
+  tm_sec    = 59
+  tm_min    = 59
+  tm_hour   = 23
+  tm_mday   = 31
+  tm_mon    = 11
+  tm_year   = 200
+  tm_wday   = 5
+  tm_yday   = 364
+  tm_isdst  = 0
+  tm_gmtoff = 0
+  tm_zone   = UTC
+
+Year 2038, pre-i32 overflow:
+  tm_sec    = 7
+  tm_min    = 14
+  tm_hour   = 3
+  tm_mday   = 19
+  tm_mon    = 0
+  tm_year   = 138
+  tm_wday   = 2
+  tm_yday   = 18
+  tm_isdst  = 0
+  tm_gmtoff = 0
+  tm_zone   = UTC
+
+Year 2038, post-i32 overflow:
+  tm_sec    = 8
+  tm_min    = 14
+  tm_hour   = 3
+  tm_mday   = 19
+  tm_mon    = 0
+  tm_year   = 138
+  tm_wday   = 2
+  tm_yday   = 18
+  tm_isdst  = 0
+  tm_gmtoff = 0
+  tm_zone   = UTC
+
+Year 2106, pre-u32 overflow:
+  tm_sec    = 15
+  tm_min    = 28
+  tm_hour   = 6
+  tm_mday   = 7
+  tm_mon    = 1
+  tm_year   = 206
+  tm_wday   = 0
+  tm_yday   = 37
+  tm_isdst  = 0
+  tm_gmtoff = 0
+  tm_zone   = UTC
+
+Year 2106, post-u32 overflow:
+  tm_sec    = 16
+  tm_min    = 28
+  tm_hour   = 6
+  tm_mday   = 7
+  tm_mon    = 1
+  tm_year   = 206
+  tm_wday   = 0
+  tm_yday   = 37
+  tm_isdst  = 0
+  tm_gmtoff = 0
+  tm_zone   = UTC

tests/time/gmtime.c

@@ -5,17 +5,109 @@
 
 #include "test_helpers.h"
 
+void print_tm(const struct tm *tm_ptr) {
+    printf("  tm_sec    = %d\n", tm_ptr->tm_sec);
+    printf("  tm_min    = %d\n", tm_ptr->tm_min);
+    printf("  tm_hour   = %d\n", tm_ptr->tm_hour);
+    printf("  tm_mday   = %d\n", tm_ptr->tm_mday);
+    printf("  tm_mon    = %d\n", tm_ptr->tm_mon);
+    printf("  tm_year   = %d\n", tm_ptr->tm_year);
+    printf("  tm_wday   = %d\n", tm_ptr->tm_wday);
+    printf("  tm_yday   = %d\n", tm_ptr->tm_yday);
+    printf("  tm_isdst  = %d\n", tm_ptr->tm_isdst);
+    printf("  tm_gmtoff = %ld\n", tm_ptr->tm_gmtoff);
+    printf("  tm_zone   = %s\n", tm_ptr->tm_zone);
+}
+
 int main(void) {
-    time_t a = 0;
-    struct tm expected = { .tm_sec = 0, .tm_min = 0, .tm_hour = 0, .tm_mday = 1, .tm_year = 70,
-                    .tm_wday = 4, .tm_yday = 0, .tm_isdst = 0, .tm_gmtoff = 0, .tm_zone = "UTC" };
-
-    struct tm *info = gmtime(&a);
-    if (info->tm_sec != expected.tm_sec || info->tm_min != expected.tm_min ||
-        info->tm_hour != expected.tm_hour || info->tm_mday != expected.tm_mday ||
-        info->tm_year != expected.tm_year || info->tm_wday != expected.tm_wday ||
-        info->tm_yday != expected.tm_yday || info->tm_isdst != expected.tm_isdst ||
-        info->tm_gmtoff != expected.tm_gmtoff || strcmp(info->tm_zone, expected.tm_zone) != 0) {
-            exit(EXIT_FAILURE);
-    }
+    time_t unix_epoch_seconds = 0;
+
+    // Exercise the different branches of the leap year logic
+    // 1970: common year
+    time_t y1970_mar01_seconds = 5097600;    // 1970-03-01 00:00:00
+    time_t y1970_dec31_seconds = 31535999;   // 1970-12-31 23:59:59
+
+    // 1972: leap year
+    time_t y1972_feb29_seconds = 68255999;   // 1972-02-29 23:59:59
+    time_t y1972_dec31_seconds = 94694399;   // 1972-12-31 23:59:59
+
+    // 2000: leap year
+    time_t y2000_feb29_seconds = 951868799;  // 2000-02-29 23:59:59
+    time_t y2000_dec31_seconds = 978307199;  // 2000-12-31 23:59:59
+
+    // 2100: common year
+    time_t y2100_mar01_seconds = 4107542400; // 2100-03-01 00:00:00
+    time_t y2100_dec31_seconds = 4133980799; // 2100-12-31 23:59:59
+
+    // Year 2038-related corner cases
+    time_t y2038_pre_seconds = 2147483647;
+    time_t y2038_post_seconds = 2147483648;
+    time_t y2106_pre_seconds = 4294967295;
+    time_t y2106_post_seconds = 4294967296;
+
+    struct tm *result_tm = NULL;
+
+    puts("Unix epoch:");
+    result_tm = gmtime(&unix_epoch_seconds);
+    print_tm(result_tm);
+
+    puts("");
+    puts("1970-03-01 00:00:00:");
+    result_tm = gmtime(&y1970_mar01_seconds);
+    print_tm(result_tm);
+
+    puts("");
+    puts("1970-12-31 23:59:59:");
+    result_tm = gmtime(&y1970_dec31_seconds);
+    print_tm(result_tm);
+
+    puts("");
+    puts("1972-02-29 23:59:59:");
+    result_tm = gmtime(&y1972_feb29_seconds);
+    print_tm(result_tm);
+
+    puts("");
+    puts("1972-12-31 23:59:59:");
+    result_tm = gmtime(&y1972_dec31_seconds);
+    print_tm(result_tm);
+
+    puts("");
+    puts("2000-02-29 23:59:59:");
+    result_tm = gmtime(&y2000_feb29_seconds);
+    print_tm(result_tm);
+
+    puts("");
+    puts("2000-12-31 23:59:59:");
+    result_tm = gmtime(&y2000_dec31_seconds);
+    print_tm(result_tm);
+
+    puts("");
+    puts("2100-03-01 00:00:00:");
+    result_tm = gmtime(&y2100_mar01_seconds);
+    print_tm(result_tm);
+
+    puts("");
+    puts("2100-12-31 23:59:59:");
+    result_tm = gmtime(&y2100_dec31_seconds);
+    print_tm(result_tm);
+
+    puts("");
+    puts("Year 2038, pre-i32 overflow:");
+    result_tm = gmtime(&y2038_pre_seconds);
+    print_tm(result_tm);
+
+    puts("");
+    puts("Year 2038, post-i32 overflow:");
+    result_tm = gmtime(&y2038_post_seconds);
+    print_tm(result_tm);
+
+    puts("");
+    puts("Year 2106, pre-u32 overflow:");
+    result_tm = gmtime(&y2106_pre_seconds);
+    print_tm(result_tm);
+
+    puts("");
+    puts("Year 2106, post-u32 overflow:");
+    result_tm = gmtime(&y2106_post_seconds);
+    print_tm(result_tm);
 }