The United States Naval Observatory is the official time source for the U.S. Department of Defense. It also determines the standard of time for the entire United States of America.
Determining sunrise and sunset is important for determining the beginning and ending of God's weekly Sabbath. The weekly Sabbath begins at sunset on Friday and continues to sunset on Saturday.
The rising and setting of the sun is also critical for determining the start and end of God's annual Feast days. The seven annual Biblical Feasts of God are Passover, Days of Unleavened Bread, Pentecost, the Day of Trumpets, the Day of Atonement, the Feast of Tabernacles, and the Last Great Day.