diff --git a/quinn/src/connection.rs b/quinn/src/connection.rs
index eee22b613..3df0159b2 100644
--- a/quinn/src/connection.rs
+++ b/quinn/src/connection.rs
@@ -952,7 +952,7 @@ pub(crate) struct State {
 
 impl State {
     fn drive_transmit(&mut self, cx: &mut Context) -> io::Result<bool> {
-        let now = Instant::now();
+        let now = self.runtime.now();
         let mut transmits = 0;
 
         let max_datagrams = self.socket.max_transmit_segments();
@@ -1170,7 +1170,7 @@ impl State {
 
         // A timer expired, so the caller needs to check for
         // new transmits, which might cause new timers to be set.
-        self.inner.handle_timeout(Instant::now());
+        self.inner.handle_timeout(self.runtime.now());
         self.timer_deadline = None;
         true
     }
@@ -1213,7 +1213,7 @@ impl State {
     }
 
     fn close(&mut self, error_code: VarInt, reason: Bytes, shared: &Shared) {
-        self.inner.close(Instant::now(), error_code, reason);
+        self.inner.close(self.runtime.now(), error_code, reason);
         self.terminate(ConnectionError::LocallyClosed, shared);
         self.wake();
     }
diff --git a/quinn/src/endpoint.rs b/quinn/src/endpoint.rs
index cf307f0df..771e835c5 100644
--- a/quinn/src/endpoint.rs
+++ b/quinn/src/endpoint.rs
@@ -197,7 +197,7 @@ impl Endpoint {
 
         let (ch, conn) = endpoint
             .inner
-            .connect(Instant::now(), config, addr, server_name)?;
+            .connect(self.runtime.now(), config, addr, server_name)?;
 
         let socket = endpoint.socket.clone();
         Ok(endpoint
@@ -324,7 +324,7 @@ impl Future for EndpointDriver {
             endpoint.driver = Some(cx.waker().clone());
         }
 
-        let now = Instant::now();
+        let now = endpoint.runtime.now();
         let mut keep_going = false;
         keep_going |= endpoint.drive_recv(cx, now)?;
         keep_going |= endpoint.handle_events(cx, &self.0.shared);
@@ -373,12 +373,11 @@ impl EndpointInner {
     ) -> Result<Connecting, ConnectionError> {
         let mut state = self.state.lock().unwrap();
         let mut response_buffer = Vec::new();
-        match state.inner.accept(
-            incoming,
-            Instant::now(),
-            &mut response_buffer,
-            server_config,
-        ) {
+        let now = state.runtime.now();
+        match state
+            .inner
+            .accept(incoming, now, &mut response_buffer, server_config)
+        {
             Ok((handle, conn)) => {
                 let socket = state.socket.clone();
                 let runtime = state.runtime.clone();
@@ -441,20 +440,21 @@ pub(crate) struct Shared {
 
 impl State {
     fn drive_recv(&mut self, cx: &mut Context, now: Instant) -> Result<bool, io::Error> {
-        self.recv_state.recv_limiter.start_cycle(Instant::now);
+        let get_time = || self.runtime.now();
+        self.recv_state.recv_limiter.start_cycle(get_time);
         if let Some(socket) = &self.prev_socket {
             // We don't care about the `PollProgress` from old sockets.
-            let poll_res = self
-                .recv_state
-                .poll_socket(cx, &mut self.inner, &**socket, now);
+            let poll_res =
+                self.recv_state
+                    .poll_socket(cx, &mut self.inner, &**socket, &*self.runtime, now);
             if poll_res.is_err() {
                 self.prev_socket = None;
             }
         };
-        let poll_res = self
-            .recv_state
-            .poll_socket(cx, &mut self.inner, &*self.socket, now);
-        self.recv_state.recv_limiter.finish_cycle(Instant::now);
+        let poll_res =
+            self.recv_state
+                .poll_socket(cx, &mut self.inner, &*self.socket, &*self.runtime, now);
+        self.recv_state.recv_limiter.finish_cycle(get_time);
         let poll_res = poll_res?;
         if poll_res.received_connection_packet {
             // Traffic has arrived on self.socket, therefore there is no need for the abandoned
@@ -712,6 +712,7 @@ impl RecvState {
         cx: &mut Context,
         endpoint: &mut proto::Endpoint,
         socket: &dyn AsyncUdpSocket,
+        runtime: &dyn Runtime,
         now: Instant,
     ) -> Result<PollProgress, io::Error> {
         let mut received_connection_packet = false;
@@ -786,7 +787,7 @@ impl RecvState {
                     return Err(e);
                 }
             }
-            if !self.recv_limiter.allow_work(Instant::now) {
+            if !self.recv_limiter.allow_work(|| runtime.now()) {
                 return Ok(PollProgress {
                     received_connection_packet,
                     keep_going: true,
diff --git a/quinn/src/mutex.rs b/quinn/src/mutex.rs
index dc276d118..c110bcec8 100644
--- a/quinn/src/mutex.rs
+++ b/quinn/src/mutex.rs
@@ -44,6 +44,8 @@ mod tracking {
         ///
         /// The purpose will be recorded in the list of last lock owners
         pub(crate) fn lock(&self, purpose: &'static str) -> MutexGuard<T> {
+            // We don't bother dispatching through Runtime::now because they're pure performance
+            // diagnostics.
             let now = Instant::now();
             let guard = self.inner.lock().unwrap();
 
diff --git a/quinn/src/runtime.rs b/quinn/src/runtime.rs
index 823c617f7..2a7a9c59f 100644
--- a/quinn/src/runtime.rs
+++ b/quinn/src/runtime.rs
@@ -19,6 +19,12 @@ pub trait Runtime: Send + Sync + Debug + 'static {
     fn spawn(&self, future: Pin<Box<dyn Future<Output = ()> + Send>>);
     /// Convert `t` into the socket type used by this runtime
     fn wrap_udp_socket(&self, t: std::net::UdpSocket) -> io::Result<Arc<dyn AsyncUdpSocket>>;
+    /// Look up the current time
+    ///
+    /// Allows simulating the flow of time for testing.
+    fn now(&self) -> Instant {
+        Instant::now()
+    }
 }
 
 /// Abstract implementation of an async timer for runtime independence
diff --git a/quinn/src/runtime/tokio.rs b/quinn/src/runtime/tokio.rs
index 42f63b214..ad321a240 100644
--- a/quinn/src/runtime/tokio.rs
+++ b/quinn/src/runtime/tokio.rs
@@ -33,6 +33,10 @@ impl Runtime for TokioRuntime {
             io: tokio::net::UdpSocket::from_std(sock)?,
         }))
     }
+
+    fn now(&self) -> Instant {
+        tokio::time::Instant::now().into_std()
+    }
 }
 
 impl AsyncTimer for Sleep {