Tosefta zu Bereschit 15:23

The number of generations which are before him, which is the end. Even though the days and the hours are like a thread of hair before ha-maqom, he counts only the generations, as it says, “He announced the generations from the start” (Is 41:4). Even though it says, “They shall be enslaved and oppressed [four hundred years]” (Gen 15:13), it also says, “And they shall return here in the fourth generation” (Gen 15:16). Indeed it says, “I will let you enjoy the full count of your days” (Ex 23:26). These are the two generations, and the remainder of the people, if he has merit they make full his years. If he did not have merit, they will take away from his years according to Rabbi Akiva. But the Sages say, If he has merit, they add to his years and if not, they take away from them. They said to Rabbi Akiva, indeed it says, “And I will add fifteen years to your life” (II Kings 20:6). He said to them, his own they added. Rabbi Akiva said to them, indeed it says, “A son shall be born to the house of David, Yoshiyahu by name” (I Kings 13:2), but Manasseh wasn’t even in the world! They said to him, does it say, “a son has been born to the House of David, son of Chizkiyah? Doesn’t it say only, “a son has been born to the House of David,” whether from Chizkiyah son of Michal of Beit David [or not]?

The number of generations which are before him, which is the end. Even though the days and the hours are like a thread of hair before ha-maqom, he counts only the generations, as it says, “He announced the generations from the start” (Is 41:4). Even though it says, “They shall be enslaved and oppressed [four hundred years]” (Gen 15:13), it also says, “And they shall return here in the fourth generation” (Gen 15:16). Indeed it says, “I will let you enjoy the full count of your days” (Ex 23:26). These are the two generations, and the remainder of the people, if he has merit they make full his years. If he did not have merit, they will take away from his years according to Rabbi Akiva. But the Sages say, If he has merit, they add to his years and if not, they take away from them. They said to Rabbi Akiva, indeed it says, “And I will add fifteen years to your life” (II Kings 20:6). He said to them, his own they added. Rabbi Akiva said to them, indeed it says, “A son shall be born to the house of David, Yoshiyahu by name” (I Kings 13:2), but Manasseh wasn’t even in the world! They said to him, does it say, “a son has been born to the House of David, son of Chizkiyah? Doesn’t it say only, “a son has been born to the House of David,” whether from Chizkiyah son of Michal of Beit David [or not]?