Happy Spring Equinox! If you're in Madison today and your thoughts turn toward the sun and its movement across the sky, you might want to visit this wonderful old sundial at Allen Centennial Gardens on the University of Wisconsin campus. And if you happen to have a sundial in your own backyard, be sure to add about five minutes to the time it reads today.
The offset has nothing to with the Spring Equinox. It's just the adjustment that has to be made this time of year for the sun's apparent figure-eight path across the sky in the course of a year, which is called an analemma. The corrections are plotted on the plaque attached to the Allen gardens sundial. Wikipedia explains the adjustment:
The orbit of the Earth is not perfectly circular and its rotational axis not perfectly perpendicular to its orbit, which together produce small variations in the sundial time throughout the year. This correction — which may be as great as 15 minutes — is described by the equation of time. A more sophisticated sundial design is required to incorporate this correction automatically; alternatively, a small plaque can be affixed to the sundial giving the offsets at various times of the year.The article has lots of links if you want to explore the subject in greater detail -- including links to analemma and the equation of time.