The significance of this pasuk (Shmos 12:40) cannot be overstated. It is our only true link between our past and our present. Many people do not realize this.
Let's investigate. Throughout all of sefer Breishis we are given a concrete, verified, Scripture-based, time-line spanning from Creation until the death of Yosef. When we count the generations up to the flood, we know that the flood occurred in 1656. When we count the generations from the flood to the birth of Avraham Avinu, we know that Avraham Avinu was born in 1948. Consequently, Yitzchok was born 100 years later in 2048 and Yaakov was born 60 years later in 2108. Yaakov immigrated to Egypt at the age of 130, or in 2238. This was 9 years after Yosef was freed from prison at the age of 30 - in 2229. Yosef died 80 years from then at the age of 110. So we know that he died in 2309.
So, as sefer Breishis comes to a close, we can confidently date our checks based on a Scriptural calendar verified by the Torah itself. Alas, as soon as sefer Shmos begins and "our eyes are darkened by the exile" we lose it all. We have no way of knowing the dates of the events in sefer Shmos (Note - we are not talking about details that Chazal tell us in the Midrashim; we are talking about dates that are unequivocally confirmed in the Torah itself - i.e., Scriptural proof). Certainly, the Torah tells us the ages of Kehas and Amram when they died and how old was Moshe when he approached Pharoah, but, for our purposes, these details are meaningless as there is no way to confirm from the Torah what year they were born. And so, our Scriptural time-line is severed. And we would have no idea what year this is...if not for 2 very essential pasukim in Tanach.
Let's look at the second one first. This pasuk is found in Melachim I 6:1. It establishes the date of the inauguration of the first Bais HaMikdash and it does so by linking the inauguration to the Exodos from Egypt:
ויהי בשמונים שנה וארבע מאות שנה לצאת בני ישראל מארץ מצרים בשנה הרביעית בחדש זו הוא החדש השני למלך שלמה על ישראל ויבן הבית ל-ה
Thus, the inauguration of the first Bais HaMikdash was 480 years after the Exodus. From this point onward we again lose our Scriptural proof (it is written but it is not unambiguously precise) and we rely on our mesora from Chazal and modern History. Our mesora from Chazal is that the first Bais HaMikdash stood 410 years followed by an exile of 70 years folllowed by the second Commonwealth of 420 years. As such, the destruction of the second Temple was (480 + 410 + 70 + 420 =) 1380 years after the Exodus. Thereupon, modern History takes over. Most historians maintain that the destruction occurred in 70 AD but we will pretend that it happened in 68 AD to compensate for a technical discrepancy of 2 years. It is now 1941 years since that date so we are now (1380 + 1941 =) 3321 years after the Exodus. Since we currently maintain that we are in the Hebrew year 5769 then it follows that the Exodus occurred 3321 years ago or (5769 - 3321 =) 2448.
And sure enough, this is our standard mesora as it is explicitly recorded in Talmud Bavli Avoda Zara 9a. Note that everything we have from the first Bais Hamikdash onward is all grounded at the time of the Exodus. And so, as long as the Exodus and Mattan Torah occurred in 2448 everything works out nicely. Likewise, it follows that suppose the Exodus and Mattan Torah did not occur in 2448, then, this year would not be 5769, would it?
But such a notion is ridiculous, isn't it? The gemara in Avoda Zara specifically tells us it was in 2448 and doesn't all of the Scriptural material that we discussed support it?
Well, not exactly. Let's check out the missing link and this is the pasuk at the head of this post (Shmos 12:40):
ומושב בני ישראל אשר ישבו במצרים שלשים שנה וארבע מאות שנה
At first glance, this pasuk is telling us in no uncertain terms how long the Jewish people lived in Egypt. What's more it is our only link to any point in sefer Breishis. The problem is that, as opposed to the numbers declared in all of the other pasukim that are referenced in this post, this is one number that we are not to take at face value.
Rashi tells us so himself. He writes (Shmos 12:40 s.v. Shloshim) that Kehas was from the immigrants to Egypt back in 2238. If you add his complete lifespan (137) with his son Amram (133) and Moshe's 80 years at the point of the Exodus, it does not amount to 430 not to mention all the overlapping years.
From a different frame of reference, if we must count 430 years from the immigration (2238), then the Exodus took place in 2668 and there is no way that this year can be 5769. And so, all accounts agree that this pasuk is not linking to the 2238 immigration.
Okay, we know where it doesn't link us. Now, where on the time line does it link us to?
Well, we all know Rashi. He tells us the well known opinion of Sefer Seder Olam that this pasuk is linking us back to 30 years before Yitzchak was born - to the Bris Bein HaBesarim. We now establish that the Bris Bein HaBesarim occurred when Avraham Avinu was 70 years old - in 2018. That was when the decree of the Egyptian servitude was invoked.
I heard a beautiful pshat (don't know the source) that the moment HKBH uttered כי גר יהיה זרעך בארץ לא להם immediately 600,000 malachim descended to Egypt to safeguard the 600,000 Jewish families that would not actually develop there until centuries later. When the Jews finally went out, these 600,000 malachim went out with them. This is the meaning of the pasuk that after 430 years, all of G-d's hosts (multitudes) went out of Egypt. It means even these hosts of malachim who were actually "enslaved" there for 430 years.
So now, we actually learn from this pasuk that the Bris Bein HaBesarim occurred at that time. But this opens up a whole new can of worms. The sequence of events that lead up to the Bris Bein HaBesarim clearly imply that it occurred after Avram immigrated to Kanaan when he was already 75 years old (2023). It is very awkward to say that this event took place five years earlier.
Numerous meforshim deal with this problem. This is a question for Parshat Lech Lecha and we will not elaborate on it here. But keep it in mind, we will recall it soon.
Thus, Rashi's explanation, which he attributes to Seder Olam, links the 430 years back to the year 2018 which places the Exodus at 2448 as the gemara in Avoda Zara asserts and our calendar is firmly established.
This explanation in Seder Olam is the root of our mesora. Seder Olam is no kleinikeit. It was written by the Tanna Rabi Yossi bar Chalafta, one of the 5 Raboseinu SheB'Darom who were ordained by Rabi Akiva. (He is the one who held his tongue when Rabi Shimon bar Yochai criticized the Romans and had to hide in the cave). Seder Olam carries the full authority of the Mishna. And so, it stands undisputed.
It only has one slight drawback and that is that we are now being forced to rely on Rabbinical commentary to resolve this issue. We cannot verify our time-line based exclusively on Scripture. And that makes it the weakest link in the chain. But as long as nobody challenges Seder Olam, we can sleep well at night knowing that this is indeed 5769 and our checks won't bounce (wishful thinking). And who would dare disturb our sleep?
Rabbeinu Moshe ben Nachman (Ramban), that's who.
Ramban (Shmos 12:40), like many others, sets out to solve the mystery of the "deceptive" link and he takes us on a long journey. He begins, as he usually does, by quoting Rashi and he promptly invokes the conflict of Avram's age as already being 75. With this, he calls upon Seder Olam who reconciles the problem by maintaining that the Bris Bein HaBesarim nevertheless occurred 5 years earlier and Avram must have made a previous visit to Eretz Kanaan.
Fair and nice. Yet, he's not buying this pshat and he's not selling it, either. Apparently, Ramban is extremely uncomfortable with the idea that an earlier event is recorded in the Torah sequentially after a later one. Without a word, he abandons Rashi and Seder Olam and proclaims an astoundingly brazen chiddush:
The mysterious 30 years come after the 400 year term of golus between Yitzchok's birth (2048) and 2448 - not before it. He initially suggests that G-d had the extra 30 years in mind up front but didn't want to tell Avram the precise number so He rounded it down to the nearest 100. Later (Shmos 12:42), he says that the original decree was initially for only 400 years but we got stuck for another 30 years "for the sinfulness of the generation". And even then, we were not worthy of redemption and were only redeemed because we "screamed out to G-d"!
In other words, G-d tacked on an extra 30 years onto the "sentence" for bad behavior!
According to Ramban, we left Egypt in 2478, not in 2448! And the Bris Bein HaBesarim occurred after Avram was 75 years old as the Torah clearly indicates. As for the famous gematria of the word "r'du" (רדו = 210 ) , Ramban will have to hold that this was the minimum span of time that we were required to be enslaved. After the first 210 years, we can be redeemed - if we are worthy.
This can also explain the motivation for the Bnei Ephraim who, according to Aggada (I don't have time to search for a source) left Egypt early and were decimated by the Plishtim. They went with the simple calculation of 400 years from the birth of Yitzchok and did not take any extra "penalty" time into consideration. So they tried to leave Egypt 30 years earlier than everyone else.
So Ramban is offering an alternative explanation to our "deceptive" link. The 430 years are not linking to 30 years prior to the birth of Yitzchok (2048), but rather from Yitzchok's birth itself. But, accordingly, the Exodus from Egypt would be on 2478 and not 2448 as is our mesora and is clearly stated in Avoda Zara 9a.
And if the Exodus was in 2478 and it is now 3321 years since the Exodus as detailed above then - this year is 5799 and not 5769!!!
Is such a thing possible? Does Ramban truly hold this to be the case?* (And, will Ramban's checks bounce?)
A chain is only as strong as its weakest link.
*Addendum - Amazingly enough, Ramban himself writes explicitly in his sefer HaEmuna V'HaBitachon (Chapter 12) the time frame of Yetziat Mitzrayim exactly in accordance with - and in the name of - Seder Olam!!