A sundial measures time by using the position of the sun in the sky. On a horizontal sundial the “style” casts a shadow onto a flat surface that is marked with the hours of the day. As the sun progresses across the sky, the shadow edge aligns with the hour-lines on the flat surface or plate indicating the time.
Sundials that function in this manner require the style being aligned with the axis of the Earth’s rotation. Hence, for the sundial to work correctly the style must point towards “true North” and not magnetic north.