The simplest and more useable clock was a Persian water clock used by manager of the qanat . The use of water clocks in Persia or Greater Iran ,especially in desert area of Iran such as Yazd, Isfahan, and Gonabad,dates back to 500 BC. Later they were also used to say the exact time of holy days of pre-Islamic religions, such as telling the time of the equal day and night ( "Nowruz)", "Chelah", or "Yaldā" – the shortest, longest, and equal-length days and nights of the years. The water clocks used in Iran were one of the most practical ancient tools for timing the yearly calendar. The water clock, or "Fenjaan", was the most accurate and commonly used timekeeping device for calculating the amount or the time that a farmer must take water from a qanat or well for irrigation, until it was replaced by more accurate current clocks. Persian water clocks were a practical and useful and necessary tool for the qanat's shareholders to calculate the length of time they could divert water to their farms or Gardens. The qanat (Kariz) was the only water source for agriculture and irrigation in arid areas so a just and fair water distribution was very important. A very fair and clever old person was elected to be the manager of the water clock called Mir Aab, and at least two full-time managers were needed to control and observe the number of Fenjans or Pengan hours and announce the exact time of the days and nights from sunrise to sunset because share holders usually were divided to the days owners and night owners. The Fenjaan consisted of a large pot full of water and a bowl with a small hole in the center. When the bowl became full of water, it would sink into the pot, and the manager would empty the bowl and again put it on the top of the water in the pot. He would record the number of times the bowl sank by putting small stones into a jar. The place where the clock was situated, and its managers, were collectively known as "khaneh Fenjaan" (time house). Usually this would be the top floor of a public-house, with west- and east-facing windows to show the time of Sunset and Sunrise. The Zibad Gonabad water clock was in use until 1965 when it was substituted by modern clocks.