Can a copyright date be used as a publication date for a web source?

Yes. If the web page you are citing does not provide a publication date, you should use the copyright date as the publication date in your works-cited-list entry, just as you would use the date on the copyright page of a book whose title page does not show a publication date. If the copyright date is given as a range of years, use only the last year in the range as the publication date.