diff --git a/parity/informant.rs b/parity/informant.rs
index 7f0534145..f2cc41f64 100644
--- a/parity/informant.rs
+++ b/parity/informant.rs
@@ -41,7 +41,9 @@ pub struct Informant {
 	skipped: AtomicUsize,
 }
 
+/// Something that can be converted to milliseconds.
 pub trait MillisecondDuration {
+	/// Get the value in milliseconds.
 	fn as_milliseconds(&self) -> u64;
 }