A long and instructive journey through time and time measuring. The reader is taken from civilizations using the moon cycles to measure time to the first solar calendars, the successive way of naming of the months and days, the secularisation of the concept of "hour" leaving the monastic rhythm to correspond to a certain amount of time indicated on the first mechanical clocks, and more and more accurate mathematical concepts and instruments of measure, since the researches go on.
No need to be a clockmaker or good at mathematics, however: everything is said in plain English, with even a pinch of suspense that keeps pushing you towards the next page.
Since the story of the calendar is closely related to that of Western social structure, the balance between religious and secular powers, and our daily life and beliefs, it also informs us on our cultural history. We discover, for instance, the importance of the long debate about when exactly Easter should be celebrated at a time when the Christian Churches (Catholic and Orthodox, mainly) had great political powers. We also see how life in growing cities of merchants imposed agreements on the calendar that would allow deadlines to be fixed and respected, and actions to be taken against those who wouldn't meet them.
The book gives a succession of key-moments or smaller events, which remind us of the extreme relativity of something we tend to take for granted - the calendar.
It is written with a pleasant fluency by an author who clearly recognizes he isn't an expert in the field but obviously researched the subject with great care. Some points are maybe treated a bit too extensively, but let's take it for a side-effect of the author's enthusiam.