A sundial needs to be constructed or adapted for the position in which it is used. If it is to have a horizontal scale, small adjustments to suit local latitude and longitude can be made by rotating the scale and altering the slant of the gnomon (that casts the shadow). But these corrections will only be valid near mid-day. Ideally a sundial will be constructed, and the scale calculated and engraved, for an exact location (which might, for example, be on a vertical wall that does not face exactly south). You need to decide whether you want Greenwich time or 'local' time, or even 'summer' time. In any case, the time displayed by a sundial will vary from the mean time during the year, up to 16 minutes ahead (November 3), and up to 14 minutes behind (February 11), only agreeing around April 15, June 13, September 1 and December 25th. Subject to these corrections a good sundial can be read to within a few minutes accuracy. By casting a point shadow (or a spot) a sundial can be made in which the scale includes the effect of the correction for variation from mean time during the year.