Instructions
Print out the Royal Copenhagen guide to dating their products which is found in the references section of this article.
Turn your vase over gently, keeping a firm grip on it so that it doesn't slip through your hands. Take note of the mark stamped on the underside of the vase and refer to the guide to determine the origin. For example, if the under-glaze stamp is just three blue wavy lines, your vase dates between approximately 1870 and1890.
Date your vase for the years going back to 1935 by referencing the green dots placed over or under one of the letters in the words Royal Copenhagen in the stamp. For example, if the dot is above the second "A," the vase was manufactured in 1946. If the dot is above both the "R" and "O," the vase dates between 1985 and 1991.