Fumiki Takahashi

TAROSKY INC.

You can enter expiration date on edit screen. 
This plugin runs cron every minute to search expired posts and make them <code>private</code>.

Taro Clockwork Post

A WordPress plugin to expire post with specified date.

Cyril Batillat

By default, WordPress can protect each post with one and only password. This plugin gives you the possibility to assign multiple passwords on each post, with an expiration date.

p5 : Plenty of Perishable Passwords for Protected Posts

Specify multiple passwords for pages / posts / custom post  types. An expiration date can be set for each password.

Taro Clockwork Post vs p5 : Plenty of Perishable Passwords for Protected Posts