diff --git a/lib/isc/Makefile.am b/lib/isc/Makefile.am
index 4f8880dfb8..df2d327213 100644
--- a/lib/isc/Makefile.am
+++ b/lib/isc/Makefile.am
@@ -61,6 +61,7 @@ libisc_la_HEADERS =			\
 	include/isc/nonce.h		\
 	include/isc/once.h		\
 	include/isc/os.h		\
+	include/isc/overflow.h		\
 	include/isc/parseint.h		\
 	include/isc/pause.h		\
 	include/isc/portset.h		\
diff --git a/lib/isc/include/isc/overflow.h b/lib/isc/include/isc/overflow.h
new file mode 100644
index 0000000000..c272cfa879
--- /dev/null
+++ b/lib/isc/include/isc/overflow.h
@@ -0,0 +1,95 @@
+/*
+ * Copyright (C) Internet Systems Consortium, Inc. ("ISC")
+ *
+ * SPDX-License-Identifier: MPL-2.0
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, you can obtain one at https://mozilla.org/MPL/2.0/.
+ *
+ * See the COPYRIGHT file distributed with this work for additional
+ * information regarding copyright ownership.
+ */
+
+#pragma once
+
+#include <isc/util.h>
+
+/*
+ * It is awkward to support signed numbers as well, so keep it simple
+ * (with a safety check).
+ */
+#define ISC_OVERFLOW_IS_UNSIGNED(a)                                      \
+	({                                                               \
+		STATIC_ASSERT((typeof(a))-1 > 0,                         \
+			      "overflow checks require unsigned types"); \
+		(a);                                                     \
+	})
+
+#define ISC_OVERFLOW_UINT_MAX(a) ISC_OVERFLOW_IS_UNSIGNED((typeof(a))-1)
+
+#define ISC_OVERFLOW_UINT_MIN(a) ISC_OVERFLOW_IS_UNSIGNED(0)
+
+/*
+ * Return true on overflow, e.g.
+ *
+ *	bool overflow = ISC_OVERFLOW_MUL(count, sizeof(array[0]), &bytes);
+ *	INSIST(!overflow);
+ */
+
+#if HAVE_BUILTIN_MUL_OVERFLOW
+#define ISC_OVERFLOW_MUL(a, b, cp) __builtin_mul_overflow(a, b, cp)
+#else
+#define ISC_OVERFLOW_MUL(a, b, cp)                                           \
+	((ISC_OVERFLOW_UINT_MAX(a) / (b) > (a)) ? (*(cp) = (a) * (b), false) \
+						: true)
+#endif
+
+#if HAVE_BUILTIN_ADD_OVERFLOW
+#define ISC_OVERFLOW_ADD(a, b, cp) __builtin_add_overflow(a, b, cp)
+#else
+#define ISC_OVERFLOW_ADD(a, b, cp)                                           \
+	((ISC_OVERFLOW_UINT_MAX(a) - (b) > (a)) ? (*(cp) = (a) + (b), false) \
+						: true)
+#endif
+
+#if HAVE_BUILTIN_SUB_OVERFLOW
+#define ISC_OVERFLOW_SUB(a, b, cp) __builtin_sub_overflow(a, b, cp)
+#else
+#define ISC_OVERFLOW_SUB(a, b, cp)                                           \
+	((ISC_OVERFLOW_UINT_MIN(a) + (b) < (a)) ? (*(cp) = (a) - (b), false) \
+						: true)
+#endif
+
+#define ISC_CHECKED_MUL(a, b)                                      \
+	({                                                         \
+		typeof(a) _c;                                      \
+		bool	  _overflow = ISC_OVERFLOW_MUL(a, b, &_c); \
+		INSIST(!_overflow);                                \
+		_c;                                                \
+	})
+
+#define ISC_CHECKED_ADD(a, b)                                      \
+	({                                                         \
+		typeof(a) _c;                                      \
+		bool	  _overflow = ISC_OVERFLOW_ADD(a, b, &_c); \
+		INSIST(!_overflow);                                \
+		_c;                                                \
+	})
+
+#define ISC_CHECKED_SUB(a, b)                                     \
+	({                                                        \
+		typeof(a) _c;                                     \
+		bool	  _overflow = ISC_OVERFLOW_SUB(a, b, cp); \
+		INSIST(!_overflow);                               \
+		_c;                                               \
+	})
+
+#define ISC_CHECKED_MUL_ADD(a, b, c)                              \
+	({                                                        \
+		size_t _d;                                        \
+		bool   _overflow = ISC_OVERFLOW_MUL(a, b, &_d) || \
+				 ISC_OVERFLOW_ADD(_d, c, &_d);    \
+		INSIST(!_overflow);                               \
+		_d;                                               \
+	})
diff --git a/lib/isc/time.c b/lib/isc/time.c
index c073061fbc..29285937ea 100644
--- a/lib/isc/time.c
+++ b/lib/isc/time.c
@@ -23,6 +23,7 @@
 #include <time.h>
 
 #include <isc/log.h>
+#include <isc/overflow.h>
 #include <isc/strerr.h>
 #include <isc/string.h>
 #include <isc/time.h>
@@ -186,16 +187,9 @@ isc_time_add(const isc_time_t *t, const isc_interval_t *i, isc_time_t *result) {
 	REQUIRE(t->nanoseconds < NS_PER_SEC && i->nanoseconds < NS_PER_SEC);
 
 	/* Seconds */
-#if HAVE_BUILTIN_ADD_OVERFLOW
-	if (__builtin_add_overflow(t->seconds, i->seconds, &result->seconds)) {
+	if (ISC_OVERFLOW_ADD(t->seconds, i->seconds, &result->seconds)) {
 		return (ISC_R_RANGE);
 	}
-#else
-	if (t->seconds > UINT_MAX - i->seconds) {
-		return (ISC_R_RANGE);
-	}
-	result->seconds = t->seconds + i->seconds;
-#endif
 
 	/* Nanoseconds */
 	result->nanoseconds = t->nanoseconds + i->nanoseconds;
@@ -217,16 +211,9 @@ isc_time_subtract(const isc_time_t *t, const isc_interval_t *i,
 	REQUIRE(t->nanoseconds < NS_PER_SEC && i->nanoseconds < NS_PER_SEC);
 
 	/* Seconds */
-#if HAVE_BUILTIN_SUB_OVERFLOW
-	if (__builtin_sub_overflow(t->seconds, i->seconds, &result->seconds)) {
+	if (ISC_OVERFLOW_SUB(t->seconds, i->seconds, &result->seconds)) {
 		return (ISC_R_RANGE);
 	}
-#else
-	if (t->seconds < i->seconds) {
-		return (ISC_R_RANGE);
-	}
-	result->seconds = t->seconds - i->seconds;
-#endif
 
 	/* Nanoseconds */
 	if (t->nanoseconds >= i->nanoseconds) {