Genesis 1: 14 And God said, Let there be lights in the firmament of the heaven to divide the day from the night; and let them be for signs, and for seasons, and for days, and years:
We only have to go back to the beginning to know that the lights in the heavens have always been there to tell time. This is how the Jewish nation determines the holy days, according to the moons and the seasons. The only one that they could not tell was the day of the beginning of the Feast of Trumpets. Leviticus 23: 24 Speak unto the children of Israel, saying, In the seventh month, in the first day of the month, shall ye have a sabbath, a memorial of blowing of trumpets, an holy convocation.
It is the only Hebrew feast to occur on a new moon. This is what Jesus meant when He spoke of the idiom: “Watch therefore, for ye know neither the day nor the hour wherein the Son of man cometh.” ~ Matthew 25:13 No one knew when the exact day or hour when the new moon occurred.