diff --git a/configure.ac b/configure.ac
index 09d59c0..0f492c1 100644
--- a/configure.ac
+++ b/configure.ac
@@ -3,6 +3,7 @@ AC_INIT([cyrus-timezones],[1.0])
 AC_CANONICAL_TARGET
 AM_INIT_AUTOMAKE
 AC_CONFIG_MACRO_DIR([m4])
+AC_CONFIG_HEADERS(config.h)
 
 dnl Checks for programs.
 AC_PROG_CC
@@ -29,7 +30,12 @@ dnl AC_FUNC_VPRINTF
 dnl AC_CHECK_FUNCS([gettimeofday memset strchr strdup strpbrk])
 
 dnl Checks for required libraries
-PKG_CHECK_MODULES([ICAL], [libical])
+PKG_CHECK_MODULES([ICAL], [libical], [
+                  dnl icalrecurrencetype_new will be new in libical 4.0
+                  AC_CHECK_LIB(ical, icalrecurrencetype_new,
+                               AC_DEFINE(HAVE_RECUR_BY_REF,[],
+                                         [Do we have support for icalrecurrencetype by reference?]))
+])
 AC_SUBST([ICAL_LIBS])
 AC_SUBST([ICAL_CFLAGS])
 PKG_CHECK_VAR([ICAL_LIBDIR], [libical], [libdir])
diff --git a/guesstz/guesstz.c b/guesstz/guesstz.c
index 36ae27d..598b6c2 100644
--- a/guesstz/guesstz.c
+++ b/guesstz/guesstz.c
@@ -57,6 +57,7 @@
 #include <jansson.h>
 #include <libical/ical.h>
 
+#include "config.h"
 #include "guesstz.h"
 
 /*
@@ -391,17 +392,24 @@ static void expand_icalobservances(icalcomponent *vtz,
         }
 
         if (rrule_prop) {
+#ifdef HAVE_RECUR_BY_REF
+            struct icalrecurrencetype *rrule =
+                icalproperty_get_rrule(rrule_prop);
+            icaltimetype *rrule_until = &rrule->until;
+#else
             struct icalrecurrencetype rrule =
                 icalproperty_get_rrule(rrule_prop);
+            icaltimetype *rrule_until = &rrule.until;
+#endif
             icalrecur_iterator *ritr = NULL;
-            unsigned eternal = icaltime_is_null_time(rrule.until);
+            unsigned eternal = icaltime_is_null_time(*rrule_until);
             unsigned trunc_until = 0;
 
             /* Check RRULE duration */
-            if (!eternal && icaltime_compare(rrule.until, start) < 0) {
+            if (!eternal && icaltime_compare(*rrule_until, start) < 0) {
                 /* RRULE ends prior to our window open -
                    check UNTIL vs tombstone */
-                obs.onset = rrule.until;
+                obs.onset = *rrule_until;
                 if (need_tomb) check_tombstone(&tombstone, &obs);
 
                 /* Remove RRULE */
@@ -411,15 +419,15 @@ static void expand_icalobservances(icalcomponent *vtz,
             else {
                 /* RRULE ends on/after our window open */
                 if (!icaltime_is_null_time(end) &&
-                    (eternal || icaltime_compare(rrule.until, end) >= 0)) {
+                    (eternal || icaltime_compare(*rrule_until, end) >= 0)) {
                     /* RRULE ends after our window close - need to adjust it */
                     trunc_until = 1;
                 }
 
                 if (!eternal) {
                     /* Adjust UNTIL to local time (for iterator) */
-                    icaltime_adjust(&rrule.until, 0, 0, 0, obs.offset_from);
-                    icaltime_set_utc(&rrule.until, 0);
+                    icaltime_adjust(rrule_until, 0, 0, 0, obs.offset_from);
+                    icaltime_set_utc(rrule_until, 0);
                 }
 
                 if (trunc_dtstart) {
@@ -470,7 +478,7 @@ static void expand_icalobservances(icalcomponent *vtz,
                         }
                         else if (!eternal) {
                             /* Set UNTIL to previous onset */
-                            rrule.until = prev_onset;
+                            *rrule_until = prev_onset;
                             icalproperty_set_rrule(rrule_prop, rrule);
                         }
 
@@ -496,7 +504,7 @@ static void expand_icalobservances(icalcomponent *vtz,
                             trunc_dtstart = 0;
 
                             /* Check if new DSTART is within 1yr of UNTIL */
-                            ydiff = rrule.until.year - recur.year;
+                            ydiff = rrule_until->year - recur.year;
                             if (!trunc_until && ydiff <= 1) {
                                 /* Remove RRULE */
                                 icalcomponent_remove_property(comp, rrule_prop);
@@ -505,7 +513,7 @@ static void expand_icalobservances(icalcomponent *vtz,
                                 if (ydiff) {
                                     /* Add UNTIL as RDATE */
                                     struct icaldatetimeperiodtype rdate = {
-                                        rrule.until,
+                                        *rrule_until,
                                         icalperiodtype_null_period()
                                     };
                                     prop = icalproperty_new_rdate(rdate);