diff --git a/mod/chat/lib.php b/mod/chat/lib.php
index 72269cf91b9..7fe83e02a9e 100644
--- a/mod/chat/lib.php
+++ b/mod/chat/lib.php
@@ -502,6 +502,9 @@ function chat_format_message_manually($message, $courseid, $sender, $currentuser
 
     // Use get_user_timezone() to find the correct timezone for displaying this message:
     // It's either the current user's timezone or else decided by some Moodle config setting
+    // First, "reset" $USER->timezone (which could have been set by a previous call to here)
+    // because otherwise the value for the previous $currentuser will take precedence over $CFG->timezone
+    $USER->timezone = 99;
     $tz = get_user_timezone($currentuser->timezone);
 
     // Before formatting the message time string, set $USER->timezone to the above.