There are two ways to do this. One is with ONTRApages. Add the Countdown Timer block to the sales page and edit the block for the proper countdown period. The page will redirect to the After expiration, redirect visitor to: URL you set in the palette:
For creating urgency in email messages or on landing page PURLS, you can also create personal coupon codes that can only be used once and have it expire 1 hour or 1 day after you generate it. Add it to a message as a merge field, and provide a link to an ONTRApage with an order form that accepts the coupon.
Check out this article for more information on coupon codes.