Tsehay, the Ge'ez and Amharic word for the sun, is spelled with this letter, and the sun is the thing the Ethiopian calendar is built on. The year has twelve months of thirty days and a thirteenth short month, Pagume, of five days or six in a leap year. The new year falls in September, and the Ethiopian year number runs seven or eight behind the Gregorian one, the difference depending on the point in the year, because the two traditions calculated the date of the Incarnation differently.
The calculation of the calendar is a discipline in its own right in the church schools. It is called bahre hasab, and it fixes the movable feasts, chiefly the date of Fasika, by rules learned as part of the traditional curriculum rather than looked up. A student who has taken that stage can work out the calendar for any year.
The Ethiopian tradition also preserves the Book of Enoch, which contains a solar calendar of its own, and the relation between that calendar and the one actually kept is a real scholarly question rather than an obvious identity. The atlas records the calendar in use, records the book, and does not run a line between them that the sources do not support.
The month names deserve a note of their own. They are not the Julian or Gregorian ones, and several are Ge'ez words with senses of their own, so a date written in Ethiopian style is not a translation of a Western date but a reading taken from a different system. Converting between the two is arithmetic rather than substitution, and getting it wrong by a day is easy.