@All - A trick to accomplish this uses the Advanced Thank You feature of Forms 2.0. Using this feature, you can check to see if a form field is in a date range and have the final URL change automatically.
So first create a custom field in Marketo called
formPostDate or whatever. You need only have one such field total in your Marketo instance.
Then create a form that uses Advanced Thank You and has
formPostDate as a hidden field. If the
formPostDate is in a certain date range, then the Thank You URL is the path to the downloadable file. If the
formPostDate is outside that range, then the Thank You URL is the path to the "Sorry" page. Like
this.
Embed the form and automatically set
formPostDate to the current date. When the form posts (which you can also do automatically) the user will go to the appropriate URL for that point in time.
Here's an example, with a download available only from March 13-15, 2015:
http://jsfiddle.net/sanford/3cbeLo8s/show/(In the example, I left the date field visible so you can change it to past the expiry date. In real use you would want it to be a hidden field.)
Note the posted date is spoofable if a lead is deeply intent on getting the content -- but remember, as long as you leave the asset in Marketo it would also be accessible via direct URL anyway. Only with HighTail-like services can you get a well and truly dead link after
x days without actually removing the underlying asset from your account.