The document introduces a new metric called Tau to measure the timeliness of data in catalogues. Tau is calculated by comparing the timestamp of when data was last updated versus the current time. The document provides three case studies calculating Tau for different data catalogues, with values ranging from 0.25 to 0.52. It encourages readers to try out Tau themselves and discuss related work, in order to build an evidence base for evaluating open data timeliness.