What's the story of "Imperial Calendar"?

In the Song Dynasty, a scholar presided over the examination. When marking the papers, he found that many scholars had written the wrong date. Asked why, the scholar replied: "Scholars can only observe the stars and the moon at night and calculate the date, which is not a day or two." Therefore, the Ming emperor studied politics and asked imperial academy to compile an almanac so that scholars could master the date. Because it was ordered by the emperor, it was called the "imperial calendar".