This is a field of the Media Form. That form is included in topics that describe media, such as newspaper articles, video, pictures, TV clips, etc. that may be of interest.
The Pub Date is the Publication Date, and is the date found on the item itself. The important thing here is for the date to be formatted in SORT ORDER FORMAT, as follows:
YYYY-MM-DD - YYYY = Year; MM=Month, DD=Day. Note that the slowest changing part of the date (year) is listed first, followed by the Month, the next most rapidly changing, and then by the Day, the most rapidly changing part. Zeros must be used to pad values that are not long enough: