He speaks of the Sabbath at the beginning of creation: “And God made in six days the works of His hands, and He completed them on the seventh day, and rested on it, and sanctified it.” Take heed, children, what He means when He says that He completed them in six days. He means this: that in six thousand years the Lord will bring all things to completion. For a day with Him signifies a thousand years. And He Himself bears me witness, saying, “Behold, the day of the Lord shall be as a thousand years.” Therefore, children, in six days—that is, in six thousand years—all things shall be brought to an end.

-Epistle of Barnabas 15:6-7