Radiocarbon dates a piece of wood, and archaeologists warn about the 'old wood problem'. What is being dated versus what is wanted?

A. The date the wood stopped exchanging carbon, its death

The method dates the sample's death rather than its use, so a beam from an old tree reused in a later building can be centuries adrift. Short-lived materials — seeds, twigs, a single year's growth — are preferred precisely because their death and their deposition are close together.