THe Torah says (Ex 12,39):
וַיֹּאפוּ אֶת־הַבָּצֵק אֲשֶׁר הוֹצִיאוּ מִמִּצְרַיִם עֻגֹת מַצּוֹת כִּי לֹא חָמֵץ כִּי־גֹרְשׁוּ מִמִּצְרַיִם וְלֹא יָכְלוּ לְהִתְמַהְמֵהַּ וְגַם־צֵדָה לֹא־עָשׂוּ לָהֶם׃
And they baked unleavened cakes of the dough that they had taken out of Egypt, for it was not leavened, since they had been driven out of Egypt and could not delay; nor had they prepared any provisions for themselves.
THe Haggadah repeats it:
מַצָּה זוֹ שֶׁאָנוֹ אוֹכְלִים, עַל שׁוּם מַה? עַל שׁוּם שֶׁלֹּא הִסְפִּיק בְּצֵקָם שֶׁל אֲבוֹתֵינוּ לְהַחֲמִיץ עַד שֶׁנִּגְלָה עֲלֵיהֶם מֶלֶךְ מַלְכֵי הַמְּלָכִים,
This matsa that we are eating, for the sake of what [is it]? For the sake [to commemorate] that our ancestors' dough was not yet able to rise, before the King of the kings of kings, the Holy One, blessed be He, revealed [Himself] to them and redeemed them.
Here's the chronological order of events I was taught:
Bney Israel had the tradition (from Avraham) the redemption will occur on the 15 of Nissan
On the first of Nissan Moses explained the drill in details, what is planned for the week of 10th to 15th of Nissan, so they started to plan the mass leaving two weeks ahead.
On the 14th they had big preparations for the last night, including the Korban and all the surrounding food. They knew they are leaving Egypt in the next 48 hours.
On the night of the 15th they had a big meal and waited another 12 hours to leave "in the middle of the day".
So from the 1st of Nissan to the noon of the 15th of Nissan when exactly they were in such hurry that they "could not delay" for 18 moments to have the dough leavened?