This variation was handled in different ways. From the start of the Western Han dynasty in 206 BC until 102 AD, "yīgēng" was moved back one "kè" every 9th day from the winter solstice to the summer solstice, and moved forward one "kè" every 9th day from summer solstice to the winter solstice. The Xia Calendar (), introduced in 102 AD, added or subtracted a "kè" to the start of night whenever the sun moved 2.5° north or south from its previous position.