diff --git a/src/public/chat.js b/src/public/chat.js
index 73ad527..5205578 100644
--- a/src/public/chat.js
+++ b/src/public/chat.js
@@ -2,7 +2,7 @@ import { genKeys, sharedKey } from "./ecdh.js";
 import { keccakAEAD } from "./aead.js";
 import { keccakKDF } from "./kdf.js";
 import { render_room, render_rooms_wrapper } from "./rooms.js";
-import { fromHexString, toHexString, generateRandomUint8Array } from "./arrayutils.js";
+import { fromHexString, toHexString, generateRandomUint8Array, concatUint8Arrays } from "./arrayutils.js";
 
 const socket = io();
 let secret = null;
@@ -44,7 +44,7 @@ socket.on('chat message', (msg, room, tag_received, iv, nonce, pubkey_received)
         render_room(room, pubkey_received);
         messages = document.getElementById(`messages-${room}`);
     }
-    const associated_data = fromHexString(Array.from((document.getElementById('pubkey')).classList).find(className => className.startsWith('key-')).replace('key-', ''));
+    const associated_data = concatUint8Arrays(fromHexString(Array.from((document.getElementById('pubkey')).classList).find(className => className.startsWith('key-')).replace('key-', '')), getRoundedUTCTime());
     const pubkey = Array.from(messages.classList).find(className => className.startsWith('key-')).replace('key-', '');
     let {plaintext, tag} = decrypt_message(msg, pubkey, fromHexString(iv), fromHexString(nonce), associated_data);
     if (tag === tag_received && pubkey == pubkey_received) {
@@ -113,7 +113,7 @@ function encrypt_message(message, user_pubkey) {
     let encoded_msg = (new TextEncoder()).encode(message);
     let iv = generateRandomUint8Array();
     let nonce = generateRandomUint8Array();
-    let associated_data = fromHexString(user_pubkey);
+    let associated_data = concatUint8Arrays(fromHexString(user_pubkey), getRoundedUTCTime());
     let {cipher, tag} = keccakAEAD.encrypt(encryption_key, encoded_msg, iv, associated_data, nonce);
     return {cipher: toHexString(cipher),
             tag: toHexString(tag),
@@ -161,3 +161,21 @@ export async function reconnectSocket() {
         console.log("Socket reconnected.");
     }, 100);
 }
+
+function getRoundedUTCTime() {
+    const roundedTime = Math.round(Date.now() / 60000) * 60000;
+    const date = new Date(roundedTime);
+
+    const year = date.getUTCFullYear();
+    const month = date.getUTCMonth();
+    const day = date.getUTCDate();
+
+    const yearHigh = Math.floor(year / 256);
+    const yearLow = year % 256;
+
+    const hours = date.getUTCHours();
+    const minutes = date.getUTCMinutes();
+
+    return new Uint8Array([yearHigh, yearLow, month, day, hours, minutes]);
+}
+