After doing some quick background research on the Calvin and Hobbes comic strip in question, I was able to find out some important details that helped me determine whether or not it is protected under US copyright law. Namely, that this strip was created and published in mid 1992 by author Bill Watterson, who is still alive to this day. Using the Copyright Genie website, I was able to enter the information I found in order to help me determine if the Calvin and Hobbes strip on the website is in violation of copyright laws.
Being that this was created after 1977, and published between march 1st 1989 and 2003, it is protected under US copyright law for the author’s life plus 70 years. In light of this, it will not become public domain until Bill Watterson passes away, and 70 years pass following his death. This means that as it has been posted on the Digital Past website, the Calvin and Hobbes comic strip is violating US copyright law.