Assume that you are a little generous and you allow the link to be valid until midnight the following day.

Use that known point in time to encode something in the URL and send it out.

When server receives URL with coding decode using midnight tomorrow. If OK then download proceeds. If not retry with midnight today and allow if that is OK. If both fail then time has expired and you can tell user so.

You will need to expand on this a little but this way you don't pass the expiry info over.