It was said to Abraham our father at the Covenant Between the Pieces: "You shall surely know that your seed shall be a stranger" (Genesis 15:13) -- this is Isaac, as it is said, "for in Isaac shall your seed be called" (Genesis 21:12). "And Isaac was sixty years old when she bore them" (Genesis 25:26). Jacob our father said, "the days of the years of my sojournings are a hundred and thirty years" (Genesis 47:9); behold, one hundred ninety years. There remained two hundred and ten -- the sign of the years of Job, and in that very period Job was born: "and Job lived after this a hundred and forty years" (Job 42:16), and it says, "and the LORD gave Job twice as much as he had before" [doubling his years to two hundred and ten] (Job 42:10). You find that when Israel went down to Egypt Job was born, and when they came up he died. Or might one think all four hundred years they were in Egypt? But behold, Kohath was among those who went down to Egypt: "and the years of the life of Kohath were a hundred and thirty-three years" (Exodus 6:18); "and the years of the life of Amram were a hundred and thirty-seven years" (Exodus 6:20); and the eighty years of Moses -- behold three hundred and fifty years. So what does Scripture teach by saying "and they shall serve them, and they shall afflict them, four hundred years" (Genesis 15:13)? As long as his seed was in a land not theirs, four hundred years. "And they shall serve them" -- these are the days of bondage; "and they shall afflict them" -- these are the days of affliction; all of them four hundred years.
When Israel Went Down to Egypt, Job Was Born
Yalkut Shimoni on Torah 77:13
נֶאֱמַר לְאַבְרָהָם אָבִינוּ בִּבְרִית בֵּין הַבְּתָרִים יָדֹעַ תֵּדַע כִּי גֵר יִהְיֶה זַרְעֲךָ זֶה יִצְחָק, שֶׁנֶּאֱמַר, (בראשית כא, יב) "כִּי בְיִצְחָק יִקָּרֵא לְךָ זָרַע". (בראשית כה, כו) "וְיִצְחָק בֶּן שִׁשִּׁים שָׁנָה בְּלֶדֶת אוֹתָם", יַעֲקֹב אָבִינוּ אָמַר "יְמֵי שְׁנֵי מְגוּרַי שְׁלֹשִׁים וּמְאַת שָׁנָה", הֲרֵי מֵאָה וְתִשְׁעִים שָׁנִים, נִשְׁתַּיֵּר מָאתַיִם וְעֶשֶׂר סִימָן שְׁנוֹתָיו שֶׁל אִיּוֹב וּבְאוֹתוֹ הַפֶּרֶק נוֹלַד אִיּוֹב, (איוב מב, טז) "וַיְחִי אִיּוֹב אַחֲרֵי זֹאת מֵאָה וְאַרְבָּעִים שָׁנָה", וְאוֹמֵר (שם י) "וַיּוֹסֵף ה' אֶת כָּל אֲשֶׁר לְאִיּוֹב לְמִשְׁנֶה" נִמְצֵאתָ אוֹמֵר כְּשֶׁיָרְדוּ יִשְׂרָאֵל לְמִצְרַיִם נוֹלַד אִיּוֹב, וּכְשֶׁעָלוּ מֵת. אוֹ יָכוֹל כָּל אַרְבַּע מֵאוֹת שָׁנָה הָיוּ בְּמִצְרַיִם, וַהֲלֹא קְהָת מִיּוֹרְדֵי מִצְרַיִם הָיָה, (שמות ו, יח) "וּשְׁנֵי חַיֵּי קְהָת שָׁלֹשׁ וּשְׁלֹשִׁים וּמְאַת שָׁנָה", "וּשְׁנֵי חַיֵּי עַמְרָם שֶׁבַע, וּשְׁלֹשִׁים וּמְאַת שָׁנָה", וּשְׁמוֹנִים שָׁנִים שֶׁל מֹשֶׁה, הֲרֵי שְׁלֹשׁ מֵאוֹת וַחֲמִשִׁים שָׁנָה וּמַה תַּלְמוּד לוֹמַר וַעֲבָדוּם וְעִנּוּ אֹתָם אַרְבַּע מֵאוֹת שָׁנָה, כָּל זְמַן שֶׁזַּרְעוֹ בְּאֶרֶץ לֹא לָהֶם אַרְבַּע מֵאוֹת שָׁנָה, וַעֲבָדוּם אֵלוּ יְמֵי הַשִּׁעְבּוּד וְעִנּוּ אֹתָם, אֵלוּ יְמֵי הָעִנּוּי, כֻּלָּן אַרְבַּע מֵאוֹת שָׁנָה.