Read the hour shown by the gnomon in the sundial. This is your local solar time. 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 or subtracted. Compare the corrected time with your watch.
The Earth revolves around its axis from west to east. Sunbeams form the steepest angle with the surface of our planet at noon. That’s the moment when the shadow cast by the gnomon is the shortest and shows north. Knowing the direction of north and dividing the dial into right sections lets us measure time as long as the Sun is not obscured by clouds.
The Earth needs 23 hours 56 minutes and 4 seconds to rotate around its axis. We treat it as 24 hours, which is why we need a correction: every four years we have a leap year with 366 days (February is 29 days long). But it gets a bit more complicated: a leap year in the Gregorian calendar is on that is divisible by 400 or divisible by 4 and not divisible by 100. For instance, 2000, 2004, 2008, 2012, 2016, 2020.
The Earth’s axis is not equal to its magnetic axis. The geographical north pole is in a different spot than the magnetic pole as you can see using the compass nearby. Also, magnetic poles move about. At the moment the distance between the geographical and the magnetic pole is around 11 degrees.