You will be given a definition of a task first, then some input of the task.
Use a disfluent question or a proper question and find the answer to the question based on the given context or return None if an answer can't be found. A disfluent question is a question that has some interruptions in it while framing. A proper question is the correct form of the question without any disfluency.

proper question: When was Gou's calendar rejected as the official calendar of the Yuan? 
 disfluent question: When did Gou's calendar become the official calendar of the Yuan no wait when was it rejected as the official calendar of the Yuan? 
 context: Guo Shoujing applied mathematics to the construction of calendars. He was one of the first mathematicians in China to work on spherical trigonometry. Gou derived a cubic interpolation formula for his astronomical calculations. His calendar, the Shoushi Li (授時暦) or Calendar for Fixing the Seasons, was disseminated in 1281 as the official calendar of the Yuan dynasty. The calendar may have been influenced solely by the work of Song dynasty astronomer Shen Kuo or possibly by the work of Arab astronomers. There are no explicit signs of Muslim influences in the Shoushi calendar, but Mongol rulers were known to be interested in Muslim calendars. Mathematical knowledge from the Middle East was introduced to China under the Mongols, and Muslim astronomers brought Arabic numerals to China in the 13th century.
Output:
None