It is wise to use cement by the date specified to avoid increasing the risk of allergic reaction to the chemicals. There is allergy protection in new cement but that quickly degrades.
That aside I think I have proven that dry cement is useable for a long time after its use by date.
I have just used some Hanson Multicem with a use by date of 18/5/15; more that 71/2 years out of date. A small amount of rendering on a bonded key has appeared to perform exactly as expected.
seven and a half years is a long time and stuff goes off quicker down here in South Devon. I reckon dry cement could last indefinitely but obviously respect has to be shown to the extra allergy risk with good ventilation and glove use, etc.
Anyone successfully used cement more out of date than mine?