Complex sentence: Viewed from the standpoint of the modern Gregorian calendar, the ancient system has many peculiar features, which is a part of its appeal: as a cultural artifact, it opens a window to the mentality of its users.
Split into simpler sentences: Viewed from the standpoint of the modern Gregorian calendar, this ancient system has many peculiar features. This is a part of its appeal: as a cultural artifact, it opens a window to the mentality of its users.