The MetaChron is an alien device of unknown origin in the possession of Trade Master Greenish of the Melnorme. It is said to predict its own destruction. When all is well, the object — which is pyramidal in shape — is white. As it progresses toward its end it turns darker and darker and when it is about to be destroyed it will be completely black.
By carefully measuring the rate at which its color changes it is possible to narrow down the time of the device's destruction to with in a month or two. Presumably this can be avoided by taking drastic action, such as taking it out of our region of space.