《長曆》是杜預據《春秋》經傳曆目而編排的曆譜。其特點是「曲循經傳月日、日食以攷晦朔」，其目標是符合經傳曆日。與杜氏之前的諸曆譜相較，《長曆》失三十三曆日、四日食，可謂最合經傳曆日。不過，除失三十三曆日外，《長曆》還失七曆日，對此，杜氏皆以「從赴」釋之（所謂從赴，指他國以崩薨禍福來告，國史據以書之）。惟因杜氏「從赴」說前後矛盾，故杜氏關於失七曆日的解釋並不可信。由杜氏據從赴、依文意置閏可知，《長曆》的編排，不單與杜氏的曆日考證相關，且與杜氏對經傳的理解相關。經學與曆學的相互印證，將有助於全面理解杜預《左氏》學。

Changli (長曆) was arranged by Duyu according to the dates in the Chunqiu and Zuozhuan. Its distinguishing feature was following these works' months, days, and solar eclipses in order to verify the dates of the new moon. Compared with the many calendars before Duyu's, Changli missed only thirty-three dates and four solar eclipse days and was mostly in accordance with the dates of Chunqiu and Zuozhuan. However, besides leaving out these thirty-three dates, Changli missed seven dates, which Duyu explained by congfu (從赴) (historians' recording of royal obituary notices). As his explanations for congfu were contradictory, we do not need to take at face value his argument regarding the missing seven dates. Through Duyu's explanations for congfu and intercalation, we know that the arrangement of Changli was affected not only by his textual research of the dates, but also by his interpretation of the classics. Comparing the classics and the calendar to each other should help us fully understand Duyu's studies of Zuozhzan.