Key:at bev:addr date
This key is used in Austria to relate to the address data set of the Austrian Federal Office for Calibration and Measurement (Bundesamt für Eich- und Vermessungswesen). It states the release date of the data set which the address of the given object was taken from in the format YYYY-MM-DD. For more information on the data set and the permission for OpenStreetMap see the corresponding Wiki page.
The JOSM plugin "Austria Adress Helper" sets this key automatically.
The current usage of the key can be viewed with Overpass Turbo.