Again, it depends where the plaster has been kept. If it's been stored in a damp free area, and you're a competent plasterer and are aware the plaster is past it's use by date, you'll manage to use it no problem at all. But if someone is just beginning to learn plastering, then don't use it. I've very often used Bonding Coat, Multi Finish etc that's been out of date for a while, but because it's been stored well, and i know it may go off quicker, i don't hang around when using it. I've never had a problem. Sometimes, an older bag of plaster is handy when doing a smaller job. It all comes down to experience really.