diff --git a/configure.ac b/configure.ac
index ef9c370..384939f 100644
--- a/configure.ac
+++ b/configure.ac
@@ -55,7 +55,10 @@ AC_TYPE_UINT32_T
 AC_CHECK_TYPE([struct timespec], [], [AC_MSG_ERROR([missing struct timespec])], [[#include <time.h>]])
 
 # Checks for functions.
-AC_CHECK_FUNCS([clock_gettime], [], [AC_MSG_ERROR([missing clock_gettime])])
+AC_CHECK_FUNCS([clock_gettime],
+    [AC_DEFINE([HAVE_CLOCK_GETTIME])],
+    [AC_CHECK_FUNCS([host_get_clock_service], [AC_DEFINE([HAVE_HOST_GET_CLOCK_SERVICE])], [AC_MSG_ERROR([no clock_gettime or host_get_clock_service])])]
+)
 
 # Checks for compiler flags.
 CCHECKFLAGS="-Wno-error"
diff --git a/libcurvecpr/lib/util.c b/libcurvecpr/lib/util.c
index 16ba18b..08fc8cc 100644
--- a/libcurvecpr/lib/util.c
+++ b/libcurvecpr/lib/util.c
@@ -5,6 +5,13 @@
 #include <curvecpr/bytes.h>
 
 #include <time.h>
+#ifdef HAVE_HOST_GET_CLOCK_SERVICE
+#include <libkern/OSAtomic.h>
+#include <mach/clock.h>
+#include <mach/mach.h>
+#include <mach/mach_error.h>
+#include <stdint.h>
+#endif
 
 #include <sodium/randombytes.h>
 
@@ -29,9 +36,37 @@ long long curvecpr_util_random_mod_n (long long n)
 /* XXX: Nanosecond granularity limits users to 1 terabyte per second. */
 long long curvecpr_util_nanoseconds (void)
 {
+    /* XXX: host_get_clock_service() has been officially deprecated for years;
+       this may need to be updated in the future. */
+#ifdef HAVE_HOST_GET_CLOCK_SERVICE
+    static int32_t cclock_registered = 0;
+    static volatile clock_serv_t cclock = 0;
+    mach_timespec_t t;
+
+    if (!cclock) {
+        if (OSAtomicCompareAndSwap32Barrier(0, 1, &cclock_registered) == 1) {
+            clock_serv_t cclock_actual;
+
+            if (host_get_clock_service(mach_host_self(), CALENDAR_CLOCK, &cclock_actual) != KERN_SUCCESS) {
+                cclock_registered = 0;
+                return -1;
+            }
+
+            cclock = cclock_actual;
+        } else {
+            while (!cclock)
+                /* Wait for clock to become available */;
+        }
+    }
+
+    if (clock_get_time(cclock, &t) != KERN_SUCCESS)
+        return -1;
+#else
     struct timespec t;
+
     if (clock_gettime(CLOCK_REALTIME, &t) != 0)
         return -1;
+#endif
 
     return t.tv_sec * 1000000000LL + t.tv_nsec;
 }
diff --git a/libcurvecpr/test/Makefile.am b/libcurvecpr/test/Makefile.am
index e64c47f..03ce746 100644
--- a/libcurvecpr/test/Makefile.am
+++ b/libcurvecpr/test/Makefile.am
@@ -13,4 +13,7 @@ messager_test_recv_requests_removal_from_sendmarkq_SOURCES = messager/test_recv_
 check_PROGRAMS += messager/test_send_with_1_failure_moves_message_from_sendq
 messager_test_send_with_1_failure_moves_message_from_sendq_SOURCES = messager/test_send_with_1_failure_moves_message_from_sendq.c
 
+check_PROGRAMS += util/test_nanoseconds
+util_test_nanoseconds_SOURCES = util/test_nanoseconds.c
+
 TESTS = $(check_PROGRAMS)
diff --git a/libcurvecpr/test/util/test_nanoseconds.c b/libcurvecpr/test/util/test_nanoseconds.c
new file mode 100644
index 0000000..28fc414
--- /dev/null
+++ b/libcurvecpr/test/util/test_nanoseconds.c
@@ -0,0 +1,22 @@
+#include <check.h>
+#include <check_extras.h>
+
+#include <curvecpr/util.h>
+
+START_TEST (test_nanoseconds)
+{
+    long long nanoseconds = curvecpr_util_nanoseconds();
+
+    fail_if(nanoseconds < 0);
+    fail_unless(nanoseconds > 1374047000000000000LL);
+
+    /* On OS X, we may cache the kernel clock reference. Make sure it still
+       works. */
+    nanoseconds = curvecpr_util_nanoseconds();
+
+    fail_if(nanoseconds < 0);
+    fail_unless(nanoseconds > 1374047000000000000LL);
+}
+END_TEST
+
+RUN_TEST (test_nanoseconds)