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The concept of expiring transforms is not actually supported. What amcl and a few others do is date their transforms in the future. This is to get around the problem that a chain of transforms is only as current as the oldest link of the chain. The future dating of a transform allows the slow transform to be looked up in the intervening period between broadcasts. This technique is dangerous in general, for it will cause tf to lag actual information, for tf will assume that information is correct until its timestamp in the future passes. However for transforms which change only very slowly this lag will not be observable, which is why it is valid for amcl to use this. It can safely be used for static transforms. I highly recommend being careful with future dating any measurements. Another technique available is to allow extrapolation. And either technique can creep up on you and cause unusual behavior which is untraceable.