--- a/browser/base/content/test/sync/browser_sync.js
+++ b/browser/base/content/test/sync/browser_sync.js
@@ -15,39 +15,48 @@ add_task(async function test_ui_state_no
 
   Services.obs.notifyObservers(null, UIState.ON_UPDATE);
   ok(called);
 
   gSync.updateAllUI = updateAllUI;
 });
 
 add_task(async function test_ui_state_signedin() {
+  const relativeDateAnchor = new Date();
   let state = {
     status: UIState.STATUS_SIGNED_IN,
     email: "foo@bar.com",
     displayName: "Foo Bar",
     avatarURL: "https://foo.bar",
     lastSync: new Date(),
     syncing: false
   };
 
+  const origRelativeTimeFormat = gSync.relativeTimeFormat;
+  gSync.relativeTimeFormat = {
+    formatBestUnit(date) {
+      return origRelativeTimeFormat.formatBestUnit(date, {now: relativeDateAnchor});
+    }
+  };
+
   gSync.updateAllUI(state);
 
   let statusBarTooltip = gSync.appMenuStatus.getAttribute("signedinTooltiptext");
   let lastSyncTooltip = gSync.formatLastSyncDate(new Date(state.lastSync));
   checkPanelUIStatusBar({
     label: "Foo Bar",
     tooltip: statusBarTooltip,
     fxastatus: "signedin",
     avatarURL: "https://foo.bar",
     syncing: false,
     syncNowTooltip: lastSyncTooltip
   });
   checkRemoteTabsPanel("PanelUI-remotetabs-main", false);
   checkMenuBarItem("sync-syncnowitem");
+  gSync.relativeTimeFormat = origRelativeTimeFormat;
 });
 
 add_task(async function test_ui_state_syncing() {
   let state = {
     status: UIState.STATUS_SIGNED_IN,
     email: "foo@bar.com",
     displayName: "Foo Bar",
     avatarURL: "https://foo.bar",