Most creams begin to lose small amounts of effective ingredients (due to time degenration) just after the expiration date - so that is why they create this date.  they assume that even if you followed all storage procedures (in terms of humidity, temperature, sunlight) the cream will begin to degrade at that point.  Though I know some medical experts who will use medications after expiration, its really not a good idea and there would be no way for you to know how effective it still is.

Since this is a steroid based cream, I assume you know that it probably is "thinning the skin" you use it on - making that skin extremely susceptible to sun damage and skin cancer.