Architecture is about designing space for people to live and work in. Horology and calendrics are about designing time systems for people to live by. They could collectively be called “time architecture.” To understand the design implications of the architecture of time requires a working knowledge of astronomy and mathematics, as well as a thorough understanding of how cultures have designed and used time throughout history. Time architecture is at the intersection of the space, the biomedical, and the social sciences.Timekeeping issues of human activities on the Moon and on Mars bring the considerations of time architecture into focus. The length of the Martian sol is close enough to that of the Earth day to serve as a useful regulator of the diurnal rhythms of humans on Mars, as well as other species we will bring with us. This is in stark contrast to the Moon’s 29-day cycle of day and night, which is far too long to serve such a purpose. Also, having an axial tilt similar to Earth’s, Mars proportionately experiences seasonal changes on approximately the same scale, albeit on a much colder end of the scale. (The seasons on Mars have been described as winter, WINTER winter, winter WINTER winter, and yet more winter.) Still, this is a factor that must be addressed in system design, and will also affect human populations on Mars, both operationally and—eventually—culturally.