You can use plaster that is a couple of months out of date because when it is bagged, they always add a margin of uncertainty to ensure that it is still useable up to the expiry date. This margin is usually 1-2 months after the expiry date.
During the manufacturing process, gypsum is heated at a high temperature to form a new compound that contains less water molecules. In essence, the bagged plaster is thirsty for more water to return it to it's original state and will therefore begin absorbing moisture from the environment where it is stored. After extended periods, bagged plaster will become stiffer due to this moisture absorption and may even clump together from the moisture partially reacting with the thirsty plaster.
Plaster also has a proportionate amount of retarder to increase the working time. During the warmer months of Spring/Summer it will contain more retarder and during the cooler months of Autumn/Winter it will contain less retarder. Therefore, if you try to use a batch that expires in Summer during Autumn/Winter, you will find that it hangs around for much longer because those batches have more retarder to increase the working time. The opposite happens for batches expiring in Winter that are used in Spring/Summer: they set too quickly because they contain less retarder.
To conclude, you should always use expired plaster with caution