--- a/toolkit/devtools/server/actors/object.js
+++ b/toolkit/devtools/server/actors/object.js
@@ -140,16 +140,22 @@ ObjectActor.prototype = {
       promiseState.value = this.hooks.createValueGrip(value);
     } else if (state == "rejected") {
       promiseState.reason = this.hooks.createValueGrip(reason);
     }
 
     promiseState.creationTimestamp = Date.now() -
       PromiseDebugging.getPromiseLifetime(rawPromise);
 
+    // If the promise is not settled, avoid adding the timeToSettle property
+    // and catch the error thrown by PromiseDebugging.getTimeToSettle.
+    try {
+      promiseState.timeToSettle = PromiseDebugging.getTimeToSettle(rawPromise);
+    } catch(e) {}
+
     return promiseState;
   },
 
   /**
    * Releases this actor from the pool.
    */
   release: function() {
     if (this.registeredPool.objectActors) {

--- a/toolkit/devtools/server/tests/unit/test_promises_actor_list_promises.js
+++ b/toolkit/devtools/server/tests/unit/test_promises_actor_list_promises.js
@@ -40,16 +40,18 @@ function* testListPromises(client, form,
   let promises = yield front.listPromises();
 
   let found = false;
   for (let p of promises) {
     equal(p.type, "object", "Expect type to be Object");
     equal(p.class, "Promise", "Expect class to be Promise");
     equal(typeof p.promiseState.creationTimestamp, "number",
       "Expect creation timestamp to be a number");
+    equal(typeof p.promiseState.timeToSettle, "number",
+      "Expect time to settle to be a number");
 
     if (p.promiseState.state === "fulfilled" &&
         p.promiseState.value === resolution) {
       found = true;
     }
   }
 
   ok(found, "Found our promise");

--- a/toolkit/devtools/server/tests/unit/test_promises_actor_onpromisesettled.js
+++ b/toolkit/devtools/server/tests/unit/test_promises_actor_onpromisesettled.js
@@ -64,22 +64,28 @@ function* testPromisesSettled(client, fo
 function oncePromiseSettled(front, resolution, resolveValue, rejectValue) {
   return new Promise(resolve => {
     events.on(front, "promises-settled", promises => {
       for (let p of promises) {
         equal(p.type, "object", "Expect type to be Object");
         equal(p.class, "Promise", "Expect class to be Promise");
         equal(typeof p.promiseState.creationTimestamp, "number",
           "Expect creation timestamp to be a number");
+        equal(typeof p.promiseState.timeToSettle, "number",
+          "Expect time to settle to be a number");
 
         if (p.promiseState.state === "fulfilled" &&
             p.promiseState.value === resolution) {
+          equal(Math.floor(p.promiseState.timeToSettle), 0,
+            "Expect time to settle for resolved promise to be 0.");
           resolve(resolveValue);
         } else if (p.promiseState.state === "rejected" &&
                    p.promiseState.reason === resolution) {
+          equal(Math.floor(p.promiseState.timeToSettle), 0,
+            "Expect time to settle for rejected promise to be 0.");
           resolve(rejectValue);
         } else {
           dump("Found non-target promise\n");
         }
       }
     });
   });
 }

new file mode 100644
--- /dev/null
+++ b/toolkit/devtools/server/tests/unit/test_promises_object_timetosettle-01.js
@@ -0,0 +1,78 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+/**
+ * Test whether or not we get the time to settle depending on the state of the
+ * promise.
+ */
+
+"use strict";
+
+const { PromisesFront } = devtools.require("devtools/server/actors/promises");
+
+let events = devtools.require("sdk/event/core");
+
+add_task(function*() {
+  let client = yield startTestDebuggerServer("test-promises-timetosettle");
+  let chromeActors = yield getChromeActors(client);
+
+  ok(Promise.toString().contains("native code"), "Expect native DOM Promise.");
+
+  yield testGetTimeToSettle(client, chromeActors, () => {
+    let p = new Promise(() => {});
+    p.name = "p";
+    let q = p.then();
+    q.name = "q";
+
+    return p;
+  });
+
+  let response = yield listTabs(client);
+  let targetTab = findTab(response.tabs, "test-promises-timetosettle");
+  ok(targetTab, "Found our target tab.");
+
+  yield testGetTimeToSettle(client, targetTab, () => {
+    const debuggee =
+      DebuggerServer.getTestGlobal("test-promises-timetosettle");
+
+    let p = new debuggee.Promise(() => {});
+    p.name = "p";
+    let q = p.then();
+    q.name = "q";
+
+    return p;
+  });
+
+  yield close(client);
+});
+
+function* testGetTimeToSettle(client, form, makePromises) {
+  let front = PromisesFront(client, form);
+
+  yield front.attach();
+  yield front.listPromises();
+
+  let onNewPromise = new Promise(resolve => {
+    events.on(front, "new-promises", promises => {
+      for (let p of promises) {
+        if (p.promiseState.state === "pending") {
+          ok(!p.promiseState.timeToSettle,
+            "Expect no time to settle for unsettled promise.");
+        } else {
+          ok(p.promiseState.timeToSettle,
+            "Expect time to settle for settled promise.");
+          equal(typeof p.promiseState.timeToSettle, "number",
+            "Expect time to settle to be a number.");
+        }
+      }
+      resolve();
+    });
+  });
+
+  let promise = makePromises();
+
+  yield onNewPromise;
+  yield front.detach();
+  // Appease eslint
+  void promise;
+}

new file mode 100644
--- /dev/null
+++ b/toolkit/devtools/server/tests/unit/test_promises_object_timetosettle-02.js
@@ -0,0 +1,69 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+/**
+ * Test that we get the expected settlement time for promise time to settle.
+ */
+
+"use strict";
+
+const { PromisesFront } = devtools.require("devtools/server/actors/promises");
+const { setTimeout } = devtools.require("sdk/timers");
+
+let events = devtools.require("sdk/event/core");
+
+add_task(function*() {
+  let client = yield startTestDebuggerServer("test-promises-timetosettle");
+  let chromeActors = yield getChromeActors(client);
+
+  ok(Promise.toString().contains("native code"), "Expect native DOM Promise.");
+
+  yield testGetTimeToSettle(client, chromeActors,
+    v => new Promise(resolve => setTimeout(() => resolve(v), 100)));
+
+  let response = yield listTabs(client);
+  let targetTab = findTab(response.tabs, "test-promises-timetosettle");
+  ok(targetTab, "Found our target tab.");
+
+  yield testGetTimeToSettle(client, targetTab, v => {
+    const debuggee =
+      DebuggerServer.getTestGlobal("test-promises-timetosettle");
+    return new debuggee.Promise(resolve => setTimeout(() => resolve(v), 100));
+  });
+
+  yield close(client);
+});
+
+function* testGetTimeToSettle(client, form, makePromise) {
+  let front = PromisesFront(client, form);
+  let resolution = "MyLittleSecret" + Math.random();
+  let found = false;
+
+  yield front.attach();
+  yield front.listPromises();
+
+  let onNewPromise = new Promise(resolve => {
+    events.on(front, "promises-settled", promises => {
+      for (let p of promises) {
+        if (p.promiseState.state === "fulfilled" &&
+            p.promiseState.value === resolution) {
+          equal(Math.floor(p.promiseState.timeToSettle / 100) * 100, 100,
+            "Expect time to settle for resolved promise to be " +
+            "approximately 100ms.");
+          found = true;
+          resolve();
+        } else {
+          dump("Found non-target promise.\n");
+        }
+      }
+    });
+  });
+
+  let promise = makePromise(resolution);
+
+  yield onNewPromise;
+  ok(found, "Found our new promise.");
+  yield front.detach();
+  // Appease eslint
+  void promise;
+}

--- a/toolkit/devtools/server/tests/unit/xpcshell.ini
+++ b/toolkit/devtools/server/tests/unit/xpcshell.ini
@@ -81,16 +81,18 @@ support-files =
 [test_eval-04.js]
 [test_eval-05.js]
 [test_promises_actor_attach.js]
 [test_promises_actor_exist.js]
 [test_promises_actor_list_promises.js]
 [test_promises_actor_onnewpromise.js]
 [test_promises_actor_onpromisesettled.js]
 [test_promises_object_creationtimestamp.js]
+[test_promises_object_timetosettle-01.js]
+[test_promises_object_timetosettle-02.js]
 [test_protocol_abort.js]
 [test_protocol_async.js]
 [test_protocol_children.js]
 [test_protocol_formtype.js]
 [test_protocol_longstring.js]
 [test_protocol_simple.js]
 [test_protocol_unregister.js]
 [test_breakpoint-01.js]