How do I cite a reference if the publication date is not known?
If a resource you are using does not list a publication date, first ask yourself whether the resource is reliable.
If you decide that it is a reputable resource, then cite it using (n.d.) to stand for "no date".
United States Sentencing Commission. (n.d.). Sourcebook of federal sentencing statistics (14th ed.). Retrieved from http://www.ussc.gov/annrpt/2009/sbtoc09.htm