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Somewhere along the line, someone realised that a slanting object would cast a more accurate shadow than a vertical one for the purposes of keeping time. The problem of seasonal changes was removed by placing the slanting object parallel to the Earth's axis. Even after clocks and watches were invented, their reliability was questionable and sundials still had to be used to check their accuracy.
But eventually, as the 18th and 19th centuries progressed, and coinciding with the era of "picturesque" or idealized landscape gardening in Britain and Europe, sundials became garden ornaments first and timekeepers second. Their value in this arena has never been questioned, as successful gardens often rely on such focal points for impact. But as modem communication and means of travel grew more sophisticated, standard time zones were adopted. As a result, your sundial will agree with your watch only on four days of the year, not because it is inaccurate but because it is measuring a different kind of time.