diff --git a/lib/isc/tests/time_test.c b/lib/isc/tests/time_test.c
index 52e0653422..22ceb90426 100644
--- a/lib/isc/tests/time_test.c
+++ b/lib/isc/tests/time_test.c
@@ -26,6 +26,111 @@
 #include <isc/time.h>
 #include <isc/util.h>
 
+#include "../time.c"
+
+#define NS_PER_S 1000000000 /*%< Nanoseconds per second. */
+#define MAX_NS	 (NS_PER_S - 1)
+
+struct time_vectors {
+	isc_time_t a;
+	isc_interval_t b;
+	isc_time_t r;
+	isc_result_t result;
+};
+
+const struct time_vectors vectors_add[8] = {
+	{ { 0, 0 }, { 0, 0 }, { 0, 0 }, ISC_R_SUCCESS },
+	{ { 0, MAX_NS }, { 0, MAX_NS }, { 1, MAX_NS - 1 }, ISC_R_SUCCESS },
+	{ { 0, NS_PER_S / 2 }, { 0, NS_PER_S / 2 }, { 1, 0 }, ISC_R_SUCCESS },
+	{ { UINT_MAX, MAX_NS }, { 0, 0 }, { UINT_MAX, MAX_NS }, ISC_R_SUCCESS },
+	{ { UINT_MAX, 0 }, { 0, MAX_NS }, { UINT_MAX, MAX_NS }, ISC_R_SUCCESS },
+	{ { UINT_MAX, 0 }, { 1, 0 }, { 0, 0 }, ISC_R_RANGE },
+	{ { UINT_MAX, MAX_NS }, { 0, 1 }, { 0, 0 }, ISC_R_RANGE },
+	{ { UINT_MAX / 2 + 1, NS_PER_S / 2 },
+	  { UINT_MAX / 2, NS_PER_S / 2 },
+	  { 0, 0 },
+	  ISC_R_RANGE },
+};
+
+const struct time_vectors vectors_sub[7] = {
+	{ { 0, 0 }, { 0, 0 }, { 0, 0 }, ISC_R_SUCCESS },
+	{ { 1, 0 }, { 0, MAX_NS }, { 0, 1 }, ISC_R_SUCCESS },
+	{ { 1, NS_PER_S / 2 },
+	  { 0, MAX_NS },
+	  { 0, NS_PER_S / 2 + 1 },
+	  ISC_R_SUCCESS },
+	{ { UINT_MAX, MAX_NS }, { UINT_MAX, 0 }, { 0, MAX_NS }, ISC_R_SUCCESS },
+	{ { 0, 0 }, { 1, 0 }, { 0, 0 }, ISC_R_RANGE },
+	{ { 0, 0 }, { 0, MAX_NS }, { 0, 0 }, ISC_R_RANGE },
+};
+
+static void
+isc_time_add_test(void **state) {
+	UNUSED(state);
+
+	for (size_t i = 0; i < ARRAY_SIZE(vectors_add); i++) {
+		isc_time_t r = { UINT_MAX, UINT_MAX };
+		isc_result_t result = isc_time_add(&(vectors_add[i].a),
+						   &(vectors_add[i].b), &r);
+		assert_int_equal(result, vectors_add[i].result);
+		if (result != ISC_R_SUCCESS) {
+			continue;
+		}
+
+		assert_int_equal(r.seconds, vectors_add[i].r.seconds);
+		assert_int_equal(r.nanoseconds, vectors_add[i].r.nanoseconds);
+	}
+
+	expect_assert_failure((void)isc_time_add(&(isc_time_t){ 0, MAX_NS + 1 },
+						 &(isc_interval_t){ 0, 0 },
+						 &(isc_time_t){ 0, 0 }));
+	expect_assert_failure((void)isc_time_add(
+		&(isc_time_t){ 0, 0 }, &(isc_interval_t){ 0, MAX_NS + 1 },
+		&(isc_time_t){ 0, 0 }));
+
+	expect_assert_failure((void)isc_time_add((isc_time_t *)NULL,
+						 &(isc_interval_t){ 0, 0 },
+						 &(isc_time_t){ 0, 0 }));
+	expect_assert_failure((void)isc_time_add(&(isc_time_t){ 0, 0 },
+						 (isc_interval_t *)NULL,
+						 &(isc_time_t){ 0, 0 }));
+	expect_assert_failure((void)isc_time_add(
+		&(isc_time_t){ 0, 0 }, &(isc_interval_t){ 0, 0 }, NULL));
+}
+
+static void
+isc_time_sub_test(void **state) {
+	UNUSED(state);
+
+	for (size_t i = 0; i < ARRAY_SIZE(vectors_sub); i++) {
+		isc_time_t r = { UINT_MAX, UINT_MAX };
+		isc_result_t result = isc_time_subtract(
+			&(vectors_sub[i].a), &(vectors_sub[i].b), &r);
+		assert_int_equal(result, vectors_sub[i].result);
+		if (result != ISC_R_SUCCESS) {
+			continue;
+		}
+		assert_int_equal(r.seconds, vectors_sub[i].r.seconds);
+		assert_int_equal(r.nanoseconds, vectors_sub[i].r.nanoseconds);
+	}
+
+	expect_assert_failure((void)isc_time_subtract(
+		&(isc_time_t){ 0, MAX_NS + 1 }, &(isc_interval_t){ 0, 0 },
+		&(isc_time_t){ 0, 0 }));
+	expect_assert_failure((void)isc_time_subtract(
+		&(isc_time_t){ 0, 0 }, &(isc_interval_t){ 0, MAX_NS + 1 },
+		&(isc_time_t){ 0, 0 }));
+
+	expect_assert_failure((void)isc_time_subtract((isc_time_t *)NULL,
+						      &(isc_interval_t){ 0, 0 },
+						      &(isc_time_t){ 0, 0 }));
+	expect_assert_failure((void)isc_time_subtract(&(isc_time_t){ 0, 0 },
+						      (isc_interval_t *)NULL,
+						      &(isc_time_t){ 0, 0 }));
+	expect_assert_failure((void)isc_time_subtract(
+		&(isc_time_t){ 0, 0 }, &(isc_interval_t){ 0, 0 }, NULL));
+}
+
 /* parse http time stamp */
 static void
 isc_time_parsehttptimestamp_test(void **state) {
@@ -295,6 +400,8 @@ isc_time_formatshorttimestamp_test(void **state) {
 int
 main(void) {
 	const struct CMUnitTest tests[] = {
+		cmocka_unit_test(isc_time_add_test),
+		cmocka_unit_test(isc_time_sub_test),
 		cmocka_unit_test(isc_time_parsehttptimestamp_test),
 		cmocka_unit_test(isc_time_formatISO8601_test),
 		cmocka_unit_test(isc_time_formatISO8601ms_test),
diff --git a/lib/isc/time.c b/lib/isc/time.c
index 77ac0da28f..83da8df0ee 100644
--- a/lib/isc/time.c
+++ b/lib/isc/time.c
@@ -53,8 +53,10 @@
  *** Intervals
  ***/
 
+#if !defined(UNIT_TESTING)
 static const isc_interval_t zero_interval = { 0, 0 };
 const isc_interval_t *const isc_interval_zero = &zero_interval;
+#endif
 
 void
 isc_interval_set(isc_interval_t *i, unsigned int seconds,
@@ -90,8 +92,10 @@ isc_interval_ms(const isc_interval_t *i) {
  *** Absolute Times
  ***/
 
+#if !defined(UNIT_TESTING)
 static const isc_time_t epoch = { 0, 0 };
 const isc_time_t *const isc_time_epoch = &epoch;
+#endif
 
 void
 isc_time_set(isc_time_t *t, unsigned int seconds, unsigned int nanoseconds) {