pesach

The Haggadah says.  ברוך הוא שהקב"ה חשב את הקץ Blessed is He that Hashem calculated the end.
What calculation was necessary?
The finalizing of the geula took place by קריעת ים סוף which was on the seventh day of pesach. Why did 'ה wait 7 days from when he took us out of mitzrayim until he finalized our redemption?
Why not do it right away? ריוסף חיים זוננפלד says the reason we had to wait until the seventh day of pesach for the final redemption, was because the galus started from the birth of יצחק which was on the first day of pesach, but his bris wasn't until the 8th day. He wasn't considered a ישראל until his bris, so the  שעבוד couldn't have been counted until after he was considered a ישראל on the 8th day.
Now if that's true why didn't 'ה just wait the 7 days & then take us out? Why do half a job & then wait a week to complete it?
The posuk says,ושמרתם את המצות כי בעצם היום וכו  you shall guard the matzos, for on this very day I will have taken your legions out of mitzrayim.
Rashi brings R' Yeshaya that says don't read the word מצות but מצוות - just as you don't let the matzos sit & become chametz don't let mitzvos sit but do them right away!
The כתב סופר asks, if we read the posuk like the drush, that it means to guard the mitzvos, how does the rest of the posuk flow that says because I took you out of mitrayim? What does that have to do with the mitzvos?
He explains with a Medrash that says that if the בני ישראל were in mitzrayim for even one instant more, they would have fallen into the final depths of tuma and couldn't have been redeemed. We see from this Medrash the importance of even one instant - it could be the difference for an entire nation to either be redeemed or be stuck in eternal servitude. It's from here our Rabbis teach us the importance of doing a mitzva right away, for if you don't do it now you never know what the next moment will bring.
Maybe that's why 'ה had to take us out at that instant in order for us to be able to get out so we didn't sink into the lowest abyss of tuma, but being that it wasn't really the time yet - it had to be 400 years to the day, He had to wait the 7 days to complete our redemption.
Teaching us that when He wants something to happen it will no matter how He has to calculate & never to miss that one second that could make all the difference in the world!