Sundials indicate the local solar time, unless otherwise corrected. To obtain the standard clock time, three types of corrections need to be made.
First, the solar time needs to be corrected for the longitude of the sundial relative to the longitude at which the official time zone is defined.
For example, a sundial located west of Greenwich meridian, Saragossa or Madrid for instance, but within the same time-zone, shows a time before the official time determined by the meridian; it will show "noon" after the official noon has passed, since the Sun passes overhead later, since the sundial is further in the west. This correction is often made by rotating the hour-lines by an angle equaling the difference in longitudes.
Second, the practice of daylight saving time shifts the official time away from solar time by an hour or, in other cases as Spain, by two. This correction can be made by numbering the hour-lines with two sets of numbers (but it is not usual).
Third, 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 vary approximately from -16 minutes to +16 minutes, is described by the so-called “equation of time E”. The equation of time is formally defined as the difference between mean solar time and true solar time. This is due to the fact that the Earth moving around the Sun verified the law of areas, resulting a non-uniform motion which mechanical watches cannot fit exactly. So the real Sun sometimes outstrip the average Sun (or theory Sun for mechanical watches) and sometimes the opposite occurs. This correction, the equation of time, appears tabulated in astronomical annual.
So in response to the above, to determine the official time reading a sundial we must apply the following formula
Official Time = Sundial Time + E ± L + (1 or 2) hours
where E is the equation of time, L the longitude of the place (taking sign - if it is to the east and taking the sign + if the west), and add 1 hour if it rules the winter time or 2 if the summer (in Spain).
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 marker of the sundial, the gnomon is set parallel to the Earth's axis and pointing at the celestial pole. Its angle therefore depends on the latitude. The shadow of the gnomon falls on a dial, allowing the time can be read off.