Copyright attaches to a work as soon as the work is fixed in a tangible form. For a blog post, this might mean when the post was originally saved in an offline text editor, which may or may not be the date the post was published on the website. The copyright for a digital photograph can attach when the picture was taken, which could be months or years before the photo was published online. Other types of Web content have similar issues; copyright can attach before the website puts it online.
Copyright holders can choose to register their copyrights with the United States Copyright Office. This does not grant the creator of the work a copyright, which she already has. It merely registers the copyright and provides additional rights, such as the ability to sue and to receive statutory damages for copyright infringement. You can determine when a copyright was registered by checking with the U.S. Copyright Office. Works registered after January 1, 1978 can be searched online. For prior works, you must go to the Library of Congress and search the card catalog. Thus, if a website has published older material, you may have to go to Washington D.C. to see when it was registered.
Not everything on a website has a copyright attached to it. For example, domain names are not copyrightable, but they can be trademarked. Things that the website did not create, such as common widgets, are not copyrightable by the website unless that site created them. Other things, such as articles, stories, pictures and databases can have copyrights attached to them. If the material is revised, then it can have more than one copyright date: the original creation date and the revision date. In the case of registered works, both the original and revised works must be registered separately.
Finding the Copyright Date
Because copyright can attach before the material on the website is published online, you may have to ask the creator when she first created the work. If you are in a legal dispute with the creator, you will have to resort to other methods. Sometimes, you can look at the material in a search engine to see a date of publication. However, that will be an approximation at best. For other content, you will need to obtain a copy of the original, electronic document so that you can look at the date of creation in the document's metadata. If the copyright date is the subject of dispute, the date is ultimately determined by a judge or a jury.