diff --git a/lib/reducers/message-reducer.js b/lib/reducers/message-reducer.js
--- a/lib/reducers/message-reducer.js
+++ b/lib/reducers/message-reducer.js
@@ -1171,7 +1171,6 @@
       },
     };
   } else if (action.type === messageStorePruneActionType) {
-    const now = Date.now();
     const messageIDsToPrune = [];
 
     const updatedThreads: { [string]: ThreadMessageInfo } = {};
@@ -1180,7 +1179,6 @@
       if (!thread) {
         continue;
       }
-      thread = { ...thread, lastPruned: now };
 
       const newMessageIDs = [...thread.messageIDs];
       const removed = newMessageIDs.splice(defaultNumberPerThread);