date: bring back -u complexity with putenv

This reverts my commit b64f9bfe4f
dated 2026-04-02.  I forgot that posixtime cares whether
-u has been specified, and gets that info via TZ.
It also uses xsetenv rather than the old putenv.
This fixes ANT-2026-LR8TONF4.
* configure.ac (LOCALTIME_CACHE): Bring this back.
* src/date.c (TZSET): Bring this back.
(main): Bring back setting TZ="UTC0" in the environment.
However, use xsetenv rather than putenv, as that’s simpler.

configure.ac

@@ -304,6 +304,54 @@ AC_CHECK_FUNCS([sigsuspend],
 
 gl_WINSIZE_IN_PTEM
 
+AC_MSG_CHECKING([whether localtime caches TZ])
+AC_CACHE_VAL([utils_cv_localtime_cache],
+[if test x$ac_cv_func_tzset = xyes; then
+AC_RUN_IFELSE([AC_LANG_SOURCE([[#include <time.h>
+#if STDC_HEADERS
+# include <stdlib.h>
+#endif
+extern char **environ;
+void unset_TZ (void)
+{
+  char **from, **to;
+  for (to = from = environ; (*to = *from); from++)
+    if (! (to[0][0] == 'T' && to[0][1] == 'Z' && to[0][2] == '='))
+      to++;
+}
+int
+main ()
+{
+  time_t now = time ((time_t *) 0);
+  int hour_GMT0, hour_unset;
+  if (putenv ("TZ=GMT0") != 0)
+    return 1;
+  hour_GMT0 = localtime (&now)->tm_hour;
+  unset_TZ ();
+  hour_unset = localtime (&now)->tm_hour;
+  if (putenv ("TZ=PST8") != 0)
+    return 1;
+  if (localtime (&now)->tm_hour == hour_GMT0)
+    return 1;
+  unset_TZ ();
+  if (localtime (&now)->tm_hour != hour_unset)
+    return 1;
+  return 0;
+}]])],
+[utils_cv_localtime_cache=no],
+[utils_cv_localtime_cache=yes],
+[# If we have tzset, assume the worst when cross-compiling.
+utils_cv_localtime_cache=yes])
+else
+        # If we lack tzset, report that localtime does not cache TZ,
+        # since we can't invalidate the cache if we don't have tzset.
+        utils_cv_localtime_cache=no
+fi])dnl
+AC_MSG_RESULT([$utils_cv_localtime_cache])
+if test $utils_cv_localtime_cache = yes; then
+  AC_DEFINE([LOCALTIME_CACHE], [1], [FIXME])
+fi
+
 # Should 'sort' link libcrypto dynamically?
 AS_CASE([$LIB_CRYPTO],
   [-lcrypto],

src/date.c

@@ -107,6 +107,12 @@ static struct option const long_options[] =
 /* flags for parse_datetime2 */
 static unsigned int parse_datetime_flags;
 
+#if LOCALTIME_CACHE
+# define TZSET tzset ()
+#else
+# define TZSET /* empty */
+#endif
+
 #ifdef _DATE_FMT
 # define DATE_FMT_LANGINFO() nl_langinfo (_DATE_FMT)
 #else
@@ -570,7 +576,7 @@ main (int argc, char **argv)
   char *reference = NULL;
   bool discarded_datestr = false;
   bool discarded_set_datestr = false;
-  char const *tzstring = NULL;
+  bool utc = false;
 
   initialize_main (&argc, &argv);
   set_program_name (argv[0]);
@@ -647,7 +653,7 @@ main (int argc, char **argv)
           set_date = true;
           break;
         case 'u':
-          tzstring = "UTC0";
+          utc = true;
           break;
         case_GETOPT_HELP_CHAR;
         case_GETOPT_VERSION_CHAR (PROGRAM_NAME, AUTHORS);
@@ -656,6 +662,15 @@ main (int argc, char **argv)
         }
     }
 
+  if (utc)
+    {
+      /* POSIX says that 'date -u' is equivalent to setting the TZ
+         environment variable, so this option should do nothing other
+         than setting TZ.  */
+      xsetenv ("TZ", "UTC0", 1);
+      TZSET;
+    }
+
   int option_specified_date = (!!datestr + !!batch_file + !!reference
                                + get_resolution);
 
@@ -726,8 +741,7 @@ main (int argc, char **argv)
 
   char *format_copy = adjust_resolution (format);
   char const *format_res = format_copy ? format_copy : format;
-  if (!tzstring)
-    tzstring = getenv ("TZ");
+  char const *tzstring = getenv ("TZ");
   timezone_t tz = tzalloc (tzstring);
 
   bool ok = true;