diff --git a/tests/run-pass/time.rs b/tests/run-pass/time.rs
index 5352630fc298..ca16f5ed5217 100644
--- a/tests/run-pass/time.rs
+++ b/tests/run-pass/time.rs
@@ -20,7 +20,7 @@ fn main() {
     let now2 = Instant::now();
     assert!(now2 > now1);
 
-    #[cfg(target_os = "linux")]
+    #[cfg(target_os = "linux")] // TODO: macOS does not support Instant subtraction
     {
         let diff = now2.duration_since(now1);
         assert!(diff.as_micros() > 0);