Update code generation

app.py

@@ -110,7 +110,7 @@ def get_today_utc():
 
 
 def get_code(username: str):
-    today = get_today_utc()
+    today = get_today_utc() - timedelta(hours=8) # offset for UTC 8:00 starting time
     date_string = today.strftime("%Y-%m-%d")
     alphabet = string.ascii_letters + string.digits
     data = f"{secret_key}||{username}||{date_string}||{secret_key}"