Byzantine time is an old method of keeping time that is now mostly extinct. Hour 0:00:00 begins daily at sunset. Due to seasonal variations in the length of a day, hour zero can vary by several hours throughout the year. It is now used in very few places, for example, the Eastern Orthodox monasteries on Mount Athos in Greece. and Mar Saba monastery in the West Bank. The Byzantine calendar is a related method of keeping dates.
References
Byzantine time Wikipedia(Text) CC BY-SA