Exact time?

Who looks at a sundial might not need it. Surely the accuracy in making a sundial creates a precise instrument, as testified by some ancient and magnificent works such as the monumental dark room Meridians built in the cathedrals, sometimes bound to check the efficacy of the Gregorian Reformation of the calendar.
A sundial may also recall another Time: think to the different hour systems which may be made on a sundial, they are an evidence of the troubles of the humanity to give a meaning to a basic but evasive concept like Time. Saint Augustin advised: "I know what is the Time until someone asks me its definition"; Time is an evanescent philosophic concept but we have always looked for its measurement. This catches a need for security and convenience: if I can measure it then it does exist and so I may understand the cycle of Nature which I belong to.
Now I can't resist to a paradox, a short-circuit of the logic, weak because bare in front of the reason, strong because it is an incentive to deep thoughts; the master is Reverend Dodgson, alias Lewis Carroll, which filled us with pleasure when he proves that a stopped clock is more accurate than one that loses a minute a day. The first clock is exactly right twice every 24 hours whereas the other clock is exactly right only once in 2 years. Ignobly I propose you the very exact dark room sundial; the great darkness of its room assure a sharped shadow and a very careful reading because nobody can observe it...