diff --git a/libc/include/semaphore.h b/libc/include/semaphore.h
index 7e3dc1c..01d685b 100644
--- a/libc/include/semaphore.h
+++ b/libc/include/semaphore.h
@@ -49,6 +49,13 @@
 int sem_init(sem_t* __sem, int __shared, unsigned int __value);
 int sem_post(sem_t* __sem);
 int sem_timedwait(sem_t* __sem, const struct timespec* __ts);
+/*
+ * POSIX only supports using sem_timedwait() with CLOCK_REALTIME, however that is typically
+ * inappropriate, since that clock can change dramatically, causing the timeout to either
+ * expire earlier or much later than intended.  This function is added to use a timespec based
+ * on CLOCK_MONOTONIC that does not suffer from this issue.
+ */
+int sem_timedwait_monotonic_np(sem_t* __sem, const struct timespec* __ts) __INTRODUCED_IN(28);
 int sem_trywait(sem_t* __sem);
 int sem_wait(sem_t* __sem);