diff --git a/polkadot/validation/src/error.rs b/polkadot/validation/src/error.rs
index d3f7f973d6..fd28080afb 100644
--- a/polkadot/validation/src/error.rs
+++ b/polkadot/validation/src/error.rs
@@ -45,6 +45,8 @@ pub enum Error {
 	/// Timer failed
 	#[display(fmt = "Timer failed: {}", _0)]
 	Timer(std::io::Error),
+	#[display(fmt = "Failed to compute deadline of now + {:?}", _0)]
+	DeadlineComputeFailure(std::time::Duration),
 	/// Unable to dispatch agreement future
 	#[display(fmt = "Unable to dispatch agreement future: {:?}", _0)]
 	Executor(futures::future::ExecuteErrorKind),
diff --git a/polkadot/validation/src/lib.rs b/polkadot/validation/src/lib.rs
index 4f4e9a2762..bcb7b45e2c 100644
--- a/polkadot/validation/src/lib.rs
+++ b/polkadot/validation/src/lib.rs
@@ -627,6 +627,14 @@ impl<C, TxApi> consensus::Proposer<Block> for Proposer<C, TxApi> where
 			last_included: initial_included,
 		};
 
+		let deadline_diff = max_duration - max_duration / 3;
+		let deadline = match Instant::now().checked_add(deadline_diff) {
+			None => return Either::Right(
+				future::err(Error::DeadlineComputeFailure(deadline_diff)),
+			),
+			Some(d) => d,
+		};
+
 		Either::Left(CreateProposal {
 			parent_hash: self.parent_hash.clone(),
 			parent_number: self.parent_number.clone(),
@@ -639,7 +647,7 @@ impl<C, TxApi> consensus::Proposer<Block> for Proposer<C, TxApi> where
 			inherent_data: Some(inherent_data),
 			inherent_digests,
 			// leave some time for the proposal finalisation
-			deadline: Instant::now() + max_duration - max_duration / 3,
+			deadline,
 		})
 	}
 }