What is the significance that 1000 years in Hashem's eyes are like a day?
Avos d'R. Nasan (Nusach 2, 42): Hashem gave to Adam ha'Rishon one day, i.e. 1000 years; he left 70 for his descendants - "Yemei Shenoseinu Vahem Shiv'im Shanah" (verse 10), so Adam ha'Rishon lived 930 years
Sanhedrin 97a #1: The world will be desolate for 1000 years. Just like one year out of seven, the land is left fallow (Shemitah), also the world is empty for 1000 out of 7000 years. It says "v'Nisgav Hashem Levado ba'Yom ha'Hu" (Yeshayah 2:11), and "Yom she'Kulo Shabbos" (below, 92:1) and "Elef Shanim b'Einecha k'Yom."
Sanhedrin 97a #2: The world will be desolate for 2000 years - "Yechayenu mi'Yomayim ba'Yom ha'Shelishi Yekimenu v'Nichyeh" (Hoshe'a 6:2).
Bereishis Rabah 8:2: The Torah existed 2,000 years before the creation - "va'Ehyeh Etzlo Amon va'Ehyeh Sha'ashu'im Yom Yom" (Mishlei 8:30).
Rashi: A thousand years for people, is like a day of Hashem. This is when a little of the night is included 1 . This is why Adam ha'Rishon died within 1000 years. Had he lived 1000, this is more than a day of Hashem 2 . Perhaps the Ashmurah is (70 years,) the difference between Adam's years and 1000. We know this only from reasoning 3 .
Radak: This refers to 1000 years in man's eyes. You also told man, what is your life? Even if you plan to live 1000 years, more than Adam ha'Rishon, it is like nothing, for you will die, and it is all like one day, or like one Ashmurah of the night, and all the years are toil.
Malbim (4-5): So You tell people, repent from [making primary] this world, for 1000 years that you live in this physical world, it will seem in your eyes like yesterday, for it passes, and it seems also like an Ashmurah of the night. There is no difference whether the past was one day or 1000 years; it is not in the present.
I.e. 1000 years are "k'Yom
What is the meaning of "k'Yom Esmol Ki Ya'avor"?
Rashi: It already passed.
