diff --git a/misc.c b/misc.c
index 7e33e65a..a81aa9e4 100644
--- a/misc.c
+++ b/misc.c
@@ -13,15 +13,32 @@ isword(char32_t wc, bool spaces_only, const char32_t *delimiters)
     return isc32graph(wc);
 }
 
+void
+timespec_add(const struct timespec *a, const struct timespec *b,
+             struct timespec *res)
+{
+    const long one_sec_in_ns = 1000000000;
+
+    res->tv_sec = a->tv_sec + b->tv_sec;
+    res->tv_nsec = a->tv_nsec + b->tv_nsec;
+    /* tv_nsec may be negative */
+    if (res->tv_nsec >= one_sec_in_ns) {
+        res->tv_sec++;
+        res->tv_nsec -= one_sec_in_ns;
+    }
+}
+
 void
 timespec_sub(const struct timespec *a, const struct timespec *b,
              struct timespec *res)
 {
+    const long one_sec_in_ns = 1000000000;
+
     res->tv_sec = a->tv_sec - b->tv_sec;
     res->tv_nsec = a->tv_nsec - b->tv_nsec;
     /* tv_nsec may be negative */
     if (res->tv_nsec < 0) {
         res->tv_sec--;
-        res->tv_nsec += 1000 * 1000 * 1000;
+        res->tv_nsec += one_sec_in_ns;
     }
 }
diff --git a/misc.h b/misc.h
index ba8b2ce7..648bb65f 100644
--- a/misc.h
+++ b/misc.h
@@ -6,4 +6,5 @@
 
 bool isword(char32_t wc, bool spaces_only, const char32_t *delimiters);
 
+void timespec_add(const struct timespec *a, const struct timespec *b, struct timespec *res);
 void timespec_sub(const struct timespec *a, const struct timespec *b, struct timespec *res);