The approach of the millennium has generated a spate of books on the history of our calendar. Here's an especially good one. According to Richards, the calendar originated in humanity's desire to track the natural phenomena on which life in primitive society depended. Seasonal cycles of crop growth and animal migrations relate to the sun; tides relate to phases of the moon; and the cycle of dark and light caused by Earth's rotation makes itself manifest every day. But the relationships among these three astronomical phenomena cannot be expressed simply. Much of the history of calendars has to do with compromises made in trying to juxtapose these three cycles. At the same time, in almost every culture, the religious impulse has imposed its overlay on the calendar. Days of the week (an arbitrary but convenient division of time) were given names related to ancient gods or to planets associated with them spiritually; each of the major religions boasts its own calendar, with its own succession of seasons and holy days. After outlining the science of calendar-making, Richards describes many calendars used over the ages, including the Mayan and Aztec, and the French Revolution's attempt to divorce timekeeping from religion. The mathematics of calendars is given due attention, especially the calculation of the day of the week for past dates, and conversions between calendars--e.g., from our Gregorian calendar to the Jewish or Mayan counterparts. A final section discusses problems caused in Western calendars by the shifting date of Easter, estimated by a complex formula, and a major bone of contention among the various denominations. Appendices provide useful astronomical constants, the names of the days of the week in sundry languages, the French Revolutionary calendar, and a glossary of technical terms. Clearly written and filled with detail, this will be a strong contender in the calendar-book sweepstakes.