Sundial Corrections

The Sun's Equation of Time

Sundial time is based on where the sun is in the sky relative to the sundial. It "doesn't care" about time zones or daylight savings time.

When comparing "clock time" to "sundial time" it is important to remember that clocks are set to a common time based on time zones, and unless you are located directly on a time zone meridian (i.e. longitude), your sundial will need an adjustment.

It is also important to remember that Earth's tilted rotation axis and non-uniform motion around the sun introduce another correction to account for when comparing one's sundial to one's watch.

Equation of Time Correction

Due to the non-circular orbit of the Earth and the tilt of the Earth's rotation axis, the sun's motion as seen from the Earth is much like a car moving along a highway. While a car may maintain an average speed (say 70 mph for example), it sometimes is a little faster and sometimes a little slower. In the case of the sun, these variations are well documented and relatively easy to calculate if one is interested in that sort of thing.

Solar Analemma

One can use the graph to determine the approximate correction to apply to your sundial (the vertical scale is in minutes of time).  If the sun is "fast", then subtract the correction to the time read on the dial. If the sun is "slow", the add the correction to the dial reading.

Longitude Correction

If you are not located on or close to your time zone meridian, adjust your dial reading by adding (if west) or subtracting (if east) four minutes of clock time for each degree of longitude you are separated from from your time zone meridian. You may then compare the adjusted dial reading to your clock time.
EXAMPLE:

I am located at 93.3° W longtitude. So when the sun is directly overhead someone located at 90° W (noon to them, as measured by the sun), the sun appears to me as still slightly to the east. This translates into 3.3° × 4 minutes of time/° = 13.2 minutes of clock time. Therefore, I need to add 13.2 minutes to the reading of my sundial if I want to compare it to my clock time (which is based on time zones and specific meridians).