Test extreme tm member values

tests/expected/time/asctime.stdout

+Thu Jan  1 00:00:00 1970
+Sun Jan  1 00:00:00 1000
+Sat Dec 31 23:59:60 9999
+Sun Jan-99 00:00:00 -999
+Sat Dec999 99:99:99 9999

tests/time/asctime.c

@@ -6,12 +6,34 @@
 #include "test_helpers.h"
 
 int main(void) {
-    time_t a = 0;
-    struct tm *time_info = gmtime(&a);
+    time_t unix_epoch = 0;
+    struct tm *unix_epoch_tm_ptr = gmtime(&unix_epoch);
 
-    char *time_string = asctime(time_info);
+    char *time_string = NULL;
 
-    if (time_string == NULL || strcmp(time_string, "Thu Jan  1 00:00:00 1970\n") != 0) {
-        exit(EXIT_FAILURE);
-    }
+    /* Min/max non-UB-causing values according to ISO C11 and newer. */
+    struct tm iso_c_min_tm = {.tm_sec = 0, .tm_min = 0, .tm_hour = 0, .tm_mday = 1, .tm_mon = 0, .tm_year = 1000-1900, .tm_wday = 0, .tm_yday = 0, .tm_isdst = 0, .tm_gmtoff = 0, .tm_zone = NULL};
+    struct tm iso_c_max_tm = {.tm_sec = 60, .tm_min = 59, .tm_hour = 23, .tm_mday = 31, .tm_mon = 11, .tm_year = 9999-1900, .tm_wday = 6, .tm_yday = 0, .tm_isdst = 0, .tm_gmtoff = 0, .tm_zone = NULL};
+
+    /* Min/max non-UB-causing values according to POSIX (issue 7). These
+     * will cause UB according to the ISO standard! */
+    struct tm posix_min_tm = {.tm_sec = 0, .tm_min = 0, .tm_hour = 0, .tm_mday = -99, .tm_mon = 0, .tm_year = -999-1900, .tm_wday = 0, .tm_yday = 0, .tm_isdst = 0, .tm_gmtoff = 0, .tm_zone = NULL};
+    struct tm posix_max_tm = {.tm_sec = 99, .tm_min = 99, .tm_hour = 99, .tm_mday = 999, .tm_mon = 11, .tm_year = 9999-1900, .tm_wday = 6, .tm_yday = 0, .tm_isdst = 0, .tm_gmtoff = 0, .tm_zone = NULL};
+
+    time_string = asctime(unix_epoch_tm_ptr);
+    printf("%s", time_string);
+
+    time_string = asctime(&iso_c_min_tm);
+    printf("%s", time_string);
+
+    time_string = asctime(&iso_c_max_tm);
+    printf("%s", time_string);
+
+    time_string = asctime(&posix_min_tm);
+    printf("%s", time_string);
+
+    time_string = asctime(&posix_max_tm);
+    printf("%s", time_string);
+
+    return 0;
 }