In the alla turca time Sun always sets at the 0’th hour and rises at the 12th hour. The hours of the alla turca time have variable length. During winters the length of 1 (alla turca) night hour is longer than the length of 1 (alla turca) day hour. During summers the opposite is true.
In the alla turca time a new day would start at sunset. This would be the 00 hours. The day was divided into two parts. Each part consisting of 12 alla turca hours. The first part from sunset to sunrise and the second part from sunrise to sunset. The night hours were of equal length and similarly the day hours were of equal length. But, as mentioned above, the length of the alla turca night hours and the alla turca day hours differed from each other depending on the season.
Who were using this time system? The Ottoman Empire.
Why? Because life revolved around the prayer times which were specified according to Sun’s position in the sky.
In this system the clocks had to be adjusted by few minutes everyday. One had to adjust for locality (latitude) as well. In the northern latitudes the nights are longer compared to southern latitudes.
Daily adjustments! This is a major disadvantage right? Well…it depends on how you look at it. The alla turca time system created jobs. Seriously!
Ottomans created horology offices (muvakkithane) to provide time keeping services. Time keepers (muvakkit) were employed to make precise adjustments to the clocks every day. Time keepers would adjust the alla turca time every day at sunset.
After the conquest of Constantinople in 1453 Ottomans established timekeeping offices in the courtyards of most mosques. Ekmelettin Ihsanoglu has a concise summary of timekeeping and related activities in the Ottoman Empire:
“The muvakkit (timekeeper) was responsible for determining prayer times. They used instruments such as the quadrant, astrolabe, sextant, octant, sundials and mechanical clocks with chronometers. The timekeeping houses, besides teaching the knowledge of timekeeping, have also been institutions where mathematics and astronomy were taught. The office of the chief astronomer was established towards the end of the fifteenth century. The chief astronomer’s most important duty was the preparation of calendars. Until the year 1800, calendars were based on Ulug Bey’s Astronomical Tables and henceforth according to Cassini’s Astronomical Tables. Astronomers were responsible for determining the beginning of fasting times [imsakiye] before the month of Ramadan and preparing horoscopes [zayije] and astronomical tables (Zics). Astronomers and occasionally their assistants would be responsible for determining propitious times such as dates of accession to the throne and declaration of war and launching of ships, and for special occasions such as births, weddings and circumcisions. The astronomer would interpret horoscopes of the sultan and his family, and statesmen, and when his interpretations came true, gifts would be bestowed upon him. Astronomers would also follow important astronomical events such as comets, earthquakes, fires and eclipses of the sun and the moon and other extraordinary events and would submit their interpretations to the palace. They were also responsible for the management of timekeeping houses. The famous observatory founded in Istanbul during the reign of Murad III (1574-1595) was under the management of Chief Astronomer Takiyeddin el-Rasid (d. 1585). A total of thirty-seven scholars undertook the position of chief astronomer up until the end of the Ottoman Empire. This institution was abolished in 1924 after the proclamation of the Republic and in its place the bashmuvakkitlik (office of the chief timekeeper) was established in 1927.” 
In the late 15’th and early 16’th centuries the timekeeping devices were sundials and water clocks in the Ottoman Empire. In Istanbul the water clock in the Beyazit Mosque was used as the official timekeeping device after 1506. In the middle of the 16’th century the timekeeping slowly migrated to mechanical clocks.
In the Ottoman Empire, the lunar Hijri calendar was used until 1839. After the Tanzimat reforms in 1839, the Rumi calendar was used. The Rumi calendar is based on the Julian calendar but starts with the year of Muhammad’s emigration (Hijra) in 622 AD.
After the collapse of the Ottoman Empire, when the Turkish Republic was started, the Gregorian calendar was adopted and the use of alla turca time was abandoned. In modern Turkey the standard time system is used.