Sundials display Apparent Solar Time, which is the Earth's natural time. It's a function of both the Earth's rotation about it's axis and it's orbital motions relative to the Sun. The Earth's rotation is (very nearly) constant, but Apparent Solar Time varies slightly from clock time because:
(A) the plane of the Earth's rotation is tilted nearly 24 degrees from the plane of the solar system (the obliquity of the ecliptic), and
(B) the Earth's orbit is elliptical, not circular.
Taken together, these two effects cause a predictable, yearly cycle in the difference between solar time and clock time, in which the shorter solar hours of one season are offset by the longer solar hours of another season. This yearly cycle of solar time is mathematically described by the Equation of Time (EOT), as shown in the graphs below. (This same information is sometimes drawn as an "analemma", the figure-8 shaped diagram shown on some globes and sundials.) It applies to all sundials.
NOTE: In order for a sundial to function as a working timepiece it must be specifically designed for (or at least adjusted for) the specific place where it will be used. In particular, the proper tilt of the gnomon is determined by the latitude of the place, and the layout of the hour marks is determined by the longitude. "Store bought" mass produced sundials are unlikely to be useful for timekeeping and should generally be considered to be only ornamental sundial replicas. This is because they are all identical to each other and are not corrected for the latitude and longitude of the individual places where they will be used.
With an astronomically correct, properly aligned sundial and the Equation of Time (and some direct sunlight!) you can easily determine the correct clock time to within a minute or two of the National Institute of Standards and Technology (NIST) atomic clocks. Simply read the sundial shadow and then add or subtract minutes as indicated for the current date. For example, on May 15 solar time is about 4 minutes ahead of clock time, so on that date you just subtract 4 minutes from your sundial reading to get clock time. Note that from the beginning of April until mid September the difference is never more than 6 minutes, and that on April 16, June 15, September 2, and December 25 the difference is zero.
There are some wonderful sundials that can display clock time directly, having a built in correction for the equation of time. These are called heliochronometers. They are generally more complicated and/or more expensive than a regular sundial, and usually have either moving mechanical parts or a series of analemmas on the timescale. I'd like to make one someday, but for now I'm still enjoying tracking the difference - using my own instruments to see the cycles of solar time with my own eyes...