Da Yan calendar (大衍历 Dà Yǎn Lì,) is a book explaining the calculations for the calendar as well as the calendar itself.

Seven sections or chapters deal with the calendar and the observations supporting it. While formulating the calendar, Yi Xing used his new measurements for the stars in the 28 xiù or constellations.

He also used the data he collected on the length of the Sun's shadow at summer solstice along the meridian and the quadratic interpolation method (不等间距二次内插法) to make corrections on observations in order to implement the dingqi to predict eclipses. The theory is called dingshuo and is based on the division of the ecliptic into 24 equal segments of 15 degrees. While Yi Xing was aware of the method and used it, he could not include it in his Da Yan calendar for political reasons. The traditionalists just couldn't stand that much change, even if it was more accurate.

In addition, Yi Xing was able to measure the actual movement of the Sun -- to measure the variations of its speed on the ecliptic. The inclusion of all of these measures made the Da Yan calendar much more accurate than the Huang Ji calendar from the Suí Dynasty (隋朝 581-618), which it replaced.

It was used from 727 to 751 and served as a benchmark for calendar makers for many dynasties afterward.

Source: http://hua.umf.maine.edu/China/astronomy/tianpage/0015H_YiXing_9275_man683727bw.html