There are two kinds of time.
Solar Time, also known as Local Apparent Time or Local True Time, is based on the position of the sun in the sky where you are. As the Earth turns, the sun appears to follow an arc through the sky. It appears on the eastern horizon at dawn; rises to its highest at 12 noon when it is due south; and then sinks to set on the western horizon. We have longer daylight and a higher sun in the summer and shorter daylight with a lower sun in the winter.
The Earth’s orbit around the sun is not a perfect circle – it is slightly eliptical. This causes its speed to vary as it travels around its orbit, and this in turn causes differences in the length of a day during the year.
This is the real organic time that we, like everything else on the Earth, have evolved to live by
Mean Time was invented to allow clocks to work. It is the time of railway timetables, factory shifts and lawyers’ fees. It is based on the fiction of a fixed day length. The natural variations in the length of a day have been evened out into mean (average) time. This day is then divided into 24 equal hours, each hour into 60 equal minutes, and so on. While this system has its uses, it also forces us into an unnatural way of life – 7 am in the winter is in reality a very different time to 7 am in the summer, even if your watch insists it is the same.
If you really must convert the solar time from your sundial into mean time, you can use an equation of time chart. Add or subtract the amount of minutes given in the chart for the relevant date to give Greenwich Mean Time. You may then need to make adjustments for your longitude.
The world is divided into Time Zones, each covering a number of degrees of longitude. The UK has one time zone; some larger countries such as the USA, Russia or China, have many. All watches within a time zone are set to the same time. Prior to the introduction of time zones, a person travelling east or west would have to alter their watch as frequently as their horse. In the UK, the noon-day sun is at its highest in Swansea (4° west) 16 minutes later than London. As travel speeded up with the railways, it was inconvenient to be continually altering watches, so time zones were introduced.
If you are more than 1° east or west of Greenwich, you may wish to adapt the equation of time chart. For each degree west of the meridian you are, move the 0 line and figures 4 minutes up. East of Greenwich, move them down. In other countries you will need to know where your meridian line is, a local astronomy society will be able to help you.
If it is summer time, you will have to add on one hour to your solar time.
This is a basic introduction to sundials and time. If you want to find out more about sundials, a good starting place is the British Sundial Society.