Or HaChaim on Exodus 12:17/אור החיים על שמות י״ב:י״ז

1 ושמרתם את המצות, "You shall guard the unleavened breads, etc." G'd told us that just as He had carefully guarded that date and had not redeemed us a single day later than the timetable He had planned, so we should be careful not to allow the dough for the unleavened bread to remain inactive a minute too long ; otherwise it might begin to rise.

2The Torah added: "you shall observe this day" to teach that not only must we be very careful in the preparation of the unleavened bread, but we must also be very careful with our calendar calculations so as not to observe the Passover on the wrong day of the month. We cannot substitute a different day for celebrating this holiday. The reason is that what occurred happened בעצם i.e. this word is to be read in both directions. The careful watch over the dough and the careful observance of the correct date are both because of what occurred on that very day. We are to emulate what G'd did at that time to the best of our ability.