The absolute deadline for eating chametz is the morning before Passover, two "seasonal hours" before midday (a "seasonal hour" is 1/12 of the time between daybreak and nightfall).

Past this time, is it forbidden to eat chametz until the close of the festival in eight days. You still have one more hour in which you can use chametz for non-eating purposes, and handle the chametz in order to sell it or destroy it.

(When the day before Passover falls on Shabbat, this means that we need to eat the Shabbat meal (for which we need two challah loaves, which are chametz) early in the day. After you've eaten the challah, carefully collect any crumbs — and any remaining pieces of chametz — and flush them down the toilet. You should now have no chametz remaining in your possession.)

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