diff --git a/lib/isc/include/isc/attributes.h b/lib/isc/include/isc/attributes.h
index eb1bb53e2e..4ed540f5a8 100644
--- a/lib/isc/include/isc/attributes.h
+++ b/lib/isc/include/isc/attributes.h
@@ -102,3 +102,9 @@
 #else
 #define ISC_ATTR_UNUSED __attribute__((__unused__))
 #endif
+
+#if __STDC_VERSION__ >= 202311L
+#define ISC_CONSTEXPR constexpr
+#else
+#define ISC_CONSTEXPR static const
+#endif
diff --git a/lib/isc/include/isc/time.h b/lib/isc/include/isc/time.h
index 3e944352b3..ad74be9270 100644
--- a/lib/isc/include/isc/time.h
+++ b/lib/isc/include/isc/time.h
@@ -18,18 +18,19 @@
 #include <inttypes.h>
 #include <time.h>
 
+#include <isc/attributes.h>
 #include <isc/lang.h>
 #include <isc/types.h>
 
 /*
  * Define various time conversion constants.
  */
-static const unsigned int MS_PER_SEC = 1000;
-static const unsigned int US_PER_MS = 1000;
-static const unsigned int NS_PER_US = 1000;
-static const unsigned int US_PER_SEC = 1000 * 1000;
-static const unsigned int NS_PER_MS = 1000 * 1000;
-static const unsigned int NS_PER_SEC = 1000 * 1000 * 1000;
+ISC_CONSTEXPR unsigned int MS_PER_SEC = 1000;
+ISC_CONSTEXPR unsigned int US_PER_MS = 1000;
+ISC_CONSTEXPR unsigned int NS_PER_US = 1000;
+ISC_CONSTEXPR unsigned int US_PER_SEC = 1000 * 1000;
+ISC_CONSTEXPR unsigned int NS_PER_MS = 1000 * 1000;
+ISC_CONSTEXPR unsigned int NS_PER_SEC = 1000 * 1000 * 1000;
 
 /*
  * ISC_FORMATHTTPTIMESTAMP_SIZE needs to be 30 in C locale and potentially