Read the time shown by the polos in the sundial. Adjust it according to the diagram in the centre of the sundial: find the right month and the corresponding point on the curve. Then from the vertical axis read the number of minutes to be added. Compare the adjusted time with your watch.
How it works
The Earth revolves around its axis from the west to the east. Sun rays reach the Earth at different angles and from different directions depending on the date and time. Knowing the rules of the Sun’s apparent motion around the sky you can find the right time scale to tell the time based on the shadow cast by the polos, following an adjustment, as long as the Sun is not covered by clouds.
In the Gregorian calendar, the most widely used calendar in the world, most years are ordinary years with 365 days. However, the earth makes one revolution around the Sun not in 365 days, but in 365 days and about 6 hours. Therefore, from time to time an additional day is needed as an adjustment, thus in the Gregorian calendar sometimes there are leap years with 366 days (February is 29 days long). But the problem is slightly more complex: a leap year in the Gregorian calendar is divisible by 400 or by 4 and not divisible by 100. For instance, 2000, 2004, 2008, 2012, 2016, 2020.
The earth’s magnetic poles and its geographical poles do not overlap. Moreover, the magnetic poles also change their position over time. Therefore, the direction indicated by a magnetic needle may be at an angle to the geographical meridian.