In the process of examining Chae Chim(蔡沈)`s 『Seojipjeon(書集傳)』critically, Park Sedang threw out an alternative calendar system, ``the first month of the year by the contemporary king``(時王之正). Park Sedang grafted the idea of 『the Chronicles of Lu(春秋)』`s great unification(大一統) onto the calendar system. Such a tendency suggests a characteristic of the medieval times and means that the calendar system matters to morals. Park Sedang`s alternative calendar system was focused on the intention that the system was applicable to anyone and in all cases. He applied the theory of human nature(人情) and the resonable influence on the world(理勢論) to his alternative calendar system to afford a philosophical basis for. Moreover, he tried to prove that his alternative calendar system was the ideal system having the historical origin from the sage kings of Ha(夏)·Eun(殷)·Ju(周). Consequently Park Sedang`s alternative calendar system could be estimated to be ahead of the others. However, he disregarded Siheon-Ryeok(時憲曆) standing for one of high-tech civilizations in those days. Such attitude toward high-tech civilizations could be estimated to show his limitations of receptive capacity obviously.