A new method to calculate the Earth's age with the time length of a day based on fossil growth lines was proposed.The day time length is a direct reflection of the Earth's rotation. The existing Mesozoic-Paleozoic fossil-daily time length data show that the day time length of this period would increase proportionally with time. According to the law of conservation of angular momentum，it is suggested that the rotation velocity of the Earth would attenuated in the same period. If this increasing trend is suitable to the whole history of the Earth，fossil-day time length data can be used to calculate the age of the Earth. The calculations show that the ages obtained vary in a range of 4.519～4.495 Ga，highly consistent with the generally accepted Earth's age of 4.54 Ga. This implies that there has been a total attenuation trend of the Earth's rotation velocity from the formation of the Earth on. The currently recognized absolute age of the Earth is substantially the age of that time point for the Earth rotation velocity to begin attenuation. From this time point forward there has been a total attenuation trend of the Earth's rotation velocity. And its result revealed the essence of currently recognized absolute age of the Earth. The age of the Earth's rotational speed decay is just the time length of the tide exists. So this paper's result of te trinity is favorable to the giant impact theory for the origin of the Moon. This age of the Earth is the last age of reformed earth（new radius，new mass，new spherical surface）.