new file mode 100644
--- /dev/null
+++ b/lib/time_rz.c
@@ -0,0 +1,336 @@
+/* Time zone functions such as tzalloc and localtime_rz
+
+   Copyright 2015 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 2, or (at your option)
+   any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License along
+   with this program; if not, see <http://www.gnu.org/licenses/>.  */
+
+/* Written by Paul Eggert.  */
+
+/* Although this module is not thread-safe, any races should be fairly
+   rare and reasonably benign.  For complete thread-safety, use a C
+   library with a working timezone_t type, so that this module is not
+   needed.  */
+
+#include <config.h>
+
+#include <time.h>
+
+#include <errno.h>
+#include <stdbool.h>
+#include <stddef.h>
+#include <stdlib.h>
+#include <string.h>
+
+#if !HAVE_TZSET
+static void tzset (void) { }
+#endif
+
+/* A time zone rule.  */
+struct tm_zone
+{
+  /* More abbreviations, should they be needed.  Their TZ_IS_SET
+     members are zero.  */
+  timezone_t next;
+
+  /* If nonzero, the rule represents the TZ environment variable set
+     to the first "abbreviation" (this may be the empty string).
+     Otherwise, it represents an unset TZ.  */
+  char tz_is_set;
+
+  /* A sequence of null-terminated strings packed next to each other.
+     The strings are followed by an extra null byte.  If TZ_IS_SET,
+     there must be at least one string and the first string (which is
+     actually a TZ environment value value) may be empty.  Otherwise
+     all strings must be nonempty.
+
+     Abbreviations are stored here because otherwise the values of
+     tm_zone and/or tzname would be dead after changing TZ and calling
+     tzset.  Abbreviations never move once allocated, and are live
+     until tzfree is called.  */
+  char abbrs[FLEXIBLE_ARRAY_MEMBER];
+};
+
+/* The approximate size to use for small allocation requests.  This is
+   the largest "small" request for the GNU C library malloc.  */
+enum { DEFAULT_MXFAST = 64 * sizeof (size_t) / 4 };
+
+/* Minimum size of the ABBRS member of struct abbr.  ABBRS is larger
+   only in the unlikely case where an abbreviation longer than this is
+   used.  */
+enum { ABBR_SIZE_MIN = DEFAULT_MXFAST - offsetof (struct tm_zone, abbrs) };
+
+static char const TZ[] = "TZ";
+
+/* Magic cookie timezone_t value, for local time.  It differs from
+   NULL and from all other timezone_t values.  Only the address
+   matters; the pointer is never dereferenced.  */
+static timezone_t local_tz = (timezone_t) 1;
+
+#if HAVE_TM_ZONE || HAVE_TZNAME
+
+/* Return true if the values A and B differ according to the rules for
+   tm_isdst: A and B differ if one is zero and the other positive.  */
+static bool
+isdst_differ (int a, int b)
+{
+  return !a != !b && 0 <= a && 0 <= b;
+}
+
+/* Return true if A and B are equal.  */
+static int
+equal_tm (const struct tm *a, const struct tm *b)
+{
+  return ! ((a->tm_sec ^ b->tm_sec)
+            | (a->tm_min ^ b->tm_min)
+            | (a->tm_hour ^ b->tm_hour)
+            | (a->tm_mday ^ b->tm_mday)
+            | (a->tm_mon ^ b->tm_mon)
+            | (a->tm_year ^ b->tm_year)
+            | isdst_differ (a->tm_isdst, b->tm_isdst));
+}
+
+#endif
+
+/* Copy to ABBRS the abbreviation at ABBR with size ABBR_SIZE (this
+   includes its trailing null byte).  Append an extra null byte to
+   mark the end of ABBRS.  */
+static void
+extend_abbrs (char *abbrs, char const *abbr, size_t abbr_size)
+{
+  memcpy (abbrs, abbr, abbr_size);
+  abbrs[abbr_size + 1] = '\0';
+}
+
+/* Return a newly allocated time zone for NAME, or NULL on failure.
+   As a special case, return a nonzero constant for wall clock time, a
+   constant that survives freeing.  */
+timezone_t
+tzalloc (char const *name)
+{
+  size_t name_size = name ? strlen (name) + 1 : 0;
+  size_t abbr_size = name_size < ABBR_SIZE_MIN ? ABBR_SIZE_MIN : name_size + 1;
+  timezone_t tz = malloc (offsetof (struct tm_zone, abbrs) + abbr_size);
+  if (tz)
+    {
+      tz->next = NULL;
+      tz->tz_is_set = !!name;
+      extend_abbrs (tz->abbrs, name, name_size);
+    }
+  return tz;
+}
+
+/* Save into TZ any nontrivial time zone abbreviation used by TM,
+   and update *TM or tzname if they point to the abbreviation.
+   Return true if successful, false (setting errno) otherwise.  */
+static bool
+save_abbr (timezone_t tz, struct tm *tm)
+{
+#if HAVE_TM_ZONE || HAVE_TZNAME
+  char const *zone = NULL;
+  char **tzname_zone = NULL;
+  char *zone_copy = (char *) "";
+# if HAVE_TM_ZONE
+  zone = tm->tm_zone;
+# endif
+# if HAVE_TZNAME
+  if (! (zone && *zone) && 0 <= tm->tm_isdst)
+    zone = *(tzname_zone = &tzname[0 < tm->tm_isdst]);
+# endif
+
+  /* No need to replace null zones, or zones within the struct tm.  */
+  if (!zone || ((char *) tm <= zone && zone < (char *) (tm + 1)))
+    return true;
+
+  if (*zone)
+    {
+      zone_copy = tz->abbrs;
+
+      while (strcmp (zone_copy, zone) != 0)
+        {
+          if (! (*zone_copy || (zone_copy == tz->abbrs && tz->tz_is_set)))
+            {
+              size_t zone_size = strlen (zone) + 1;
+              if (zone_size < tz->abbrs + ABBR_SIZE_MIN - zone_copy)
+                extend_abbrs (zone_copy, zone, zone_size);
+              else
+                {
+                  tz = tz->next = tzalloc (zone);
+                  if (!tz)
+                    return false;
+                  tz->tz_is_set = 0;
+                  zone_copy = tz->abbrs;
+                }
+              break;
+            }
+
+          zone_copy += strlen (zone_copy) + 1;
+          if (!*zone_copy && tz->next)
+            {
+              tz = tz->next;
+              zone_copy = tz->abbrs;
+            }
+        }
+    }
+
+  /* Replace the zone name so that its lifetime matches that of TZ.  */
+# if HAVE_TM_ZONE
+  if (!tzname_zone)
+    tm->tm_zone = zone_copy;
+# endif
+# if HAVE_TZNAME
+  if (tzname_zone)
+    *tzname_zone = zone_copy;
+# endif
+#endif
+  return true;
+}
+
+/* Free a time zone.  */
+void
+tzfree (timezone_t tz)
+{
+  if (tz != local_tz)
+    while (tz)
+      {
+        timezone_t next = tz->next;
+        free (tz);
+        tz = next;
+      }
+}
+
+/* Get and set the TZ environment variable.  These functions can be
+   overridden by programs like Emacs that manage their own environment.  */
+
+#ifndef getenv_TZ
+static char *
+getenv_TZ (void)
+{
+  return getenv (TZ);
+}
+#endif
+
+#ifndef setenv_TZ
+static int
+setenv_TZ (char const *tz)
+{
+  return tz ? setenv (TZ, tz, 1) : unsetenv (TZ);
+}
+#endif
+
+/* Change the environment to match the specified timezone_t value.
+   Return true if successful, false (setting errno) otherwise.  */
+static bool
+change_env (timezone_t tz)
+{
+  if (setenv_TZ (tz->tz_is_set ? tz->abbrs : NULL) != 0)
+    return false;
+  tzset ();
+  return true;
+}
+
+/* Temporarily set the time zone to TZ, which must not be null.
+   Return LOCAL_TZ if the time zone setting is already correct.
+   Otherwise return a newly allocated time zone representing the old
+   setting, or NULL (setting errno) on failure.  */
+static timezone_t
+set_tz (timezone_t tz)
+{
+  char *env_tz = getenv_TZ ();
+  if (env_tz
+      ? tz->tz_is_set && strcmp (tz->abbrs, env_tz) == 0
+      : !tz->tz_is_set)
+    return local_tz;
+  else
+    {
+      timezone_t old_tz = tzalloc (env_tz);
+      if (!old_tz)
+        return old_tz;
+      if (! change_env (tz))
+        {
+          int saved_errno = errno;
+          tzfree (old_tz);
+          errno = saved_errno;
+          return NULL;
+        }
+      return old_tz;
+    }
+}
+
+/* Restore an old setting returned by set_tz.  It must not be null.
+   Return true (preserving errno) if successful, false (setting errno)
+   otherwise.  */
+static bool
+revert_tz (timezone_t tz)
+{
+  if (tz == local_tz)
+    return true;
+  else
+    {
+      int saved_errno = errno;
+      bool ok = change_env (tz);
+      if (!ok)
+        saved_errno = errno;
+      tzfree (tz);
+      errno = saved_errno;
+      return ok;
+    }
+}
+
+/* Use time zone TZ to compute localtime_r (T, TM).  */
+struct tm *
+localtime_rz (timezone_t tz, time_t const *t, struct tm *tm)
+{
+  if (!tz)
+    return gmtime_r (t, tm);
+  else
+    {
+      timezone_t old_tz = set_tz (tz);
+      if (old_tz)
+        {
+          tm = localtime_r (t, tm);
+          if (tm && !save_abbr (tz, tm))
+            tm = NULL;
+          if (revert_tz (old_tz))
+            return tm;
+        }
+      return NULL;
+    }
+}
+
+/* Use time zone TZ to compute mktime (TM).  */
+time_t
+mktime_z (timezone_t tz, struct tm *tm)
+{
+  if (!tz)
+    return timegm (tm);
+  else
+    {
+      timezone_t old_tz = set_tz (tz);
+      if (old_tz)
+        {
+          time_t t = mktime (tm);
+#if HAVE_TM_ZONE || HAVE_TZNAME
+          time_t badtime = -1;
+          struct tm tm_1;
+          if ((t != badtime
+               || (localtime_r (&t, &tm_1) && equal_tm (tm, &tm_1)))
+              && !save_abbr (tz, tm))
+            t = badtime;
+#endif
+          if (revert_tz (old_tz))
+            return t;
+        }
+      return -1;
+    }
+}