This article explains how Overton handles errors in publication dates.
By default Overton collects the publication date of each policy document either from its source website or the PDF of the policy document itself.
Occasionally this doesn’t work and we’re unable to determine what the correct publication date is. This can happen if the date is in a format we don’t recognize, or just missing, or seems otherwise unreasonable (for example it is pre-1900, or far in the future).
In these cases Overton sets the publication date field to “unknown date”. Documents with an unknown date typically appear last in the result set when sorting by publication date and are not listed when filtering or reporting by year.
The date will be blank in Excel and CSV outputs.