Time is a very important concept in Gadaa and therefore in Oromo life. Gadaa itself can be narrowly defined as a given set of time (period) which groups of individuals perform specific duties in a society. Gadaa could also mean age. The lives of individuals, rituals, ceremonies, political and economic activities are scheduled rather precisely. For this purpose, the Oromo have a calendar. The calendar is also used for weather forecasting and divination purposes.

The Oromo calendar is based on astronomical observations of the moon in conjunction with seven or eight particular stars or star groups (Legesse, 1973 and Bassi, 1988) called Urji Dhaha (guiding stars). According to this calendar system, there are approximately 30 days in a month and 12 months in a year. The first day of a month is the day the new moon appears. A day (24 hours) starts and ends at sunrise.

In the Oromo calendar each day of the month and each month of the year has a name. Instead of the expected 29 or 30 names for days of a month, there are only 27 names. These 27 days of the month are permutated through the twelve months, in such a way that the beginning of each month moves forward by 2 or 3 days. The loss per month is then the difference between the 27-day month and the 30-day month, (Legesse, 1973). One interesting observation is that, as illustrated in the computing of time like in the Oromo calendar, Oromos visualization of events is cyclical just as many events in nature are cyclical.

Since each day (called ayyaana) of a month has a name, the Oromo traditionally had no use for names of the days of a week. Perhaps it is because of this that today in different parts of Oromia different names are in use for the days of a week.

Each of the 27 days (ayyaana) of the month have special meaning and connotation to the Oromo time-keeping experts, called ayyaantu. Ayyaantu can tell the day, the month, the year and the Gadaa period by keeping track of time astronomically. They are experts, in astronomy and supplement their memory of things by examining the relative position of eight stars or star groups, (Bassi, 1988) and the moon to determine the day (ayyaana) and the month. On the basis of astronomical observations, they make an adjustment in the day name every two or three months.

The pillars found a few years ago in north-western Kenya by Lynch and Robbins (1978) has been suggested to represent a site used to develop the Oromo calendar system. According to these researchers, it is the first archaeo-astronomical evidence in subSaharan Africa. Doyle (1986) has suggested 300 B.C. as the approximate date of its invention.

According to Asmarom Legesse (1973), "The Oromo calendar is a great and unique invention and has been recorded only in a very few cultures in history of mankind." The only other known cultures with this type of time-keeping are the Chinese, Mayans and Hindus. Legesse states that the Oromo are unusual in that they seem to be the only people with a reasonably accurate calendar which ignore the sun.