Two verses in the Torah appear to contradict each other on a basic question: how many days must one eat matzah during Passover? One verse says six days. Another says seven. The Mekhilta resolves this tension with an answer rooted in agricultural law.
The distinction turns on the difference between the new crop and the old crop. If a person wants to eat matzah made from the new harvest — grain that grew during the current agricultural year — he can do so for only six of the seven days. The reason is that on the first day of Passover, grain from the new crop is still forbidden. It remains prohibited until after the omer offering is brought, as prescribed in (Leviticus 23:15). Only once that offering has been made does the new grain become permitted.
Matzah made from the old crop, however — grain harvested in a previous year — faces no such restriction. It was already permitted before Passover began. A person using old grain may eat matzah on all seven days without interruption.
The solution is elegant in its simplicity. The Torah is not contradicting itself. It is speaking to two different situations simultaneously. "Six" applies to the new crop; "seven" applies to the old. Both statements are true, and neither cancels the other.
This type of harmonization — finding that two seemingly conflicting verses address different cases — is one of the foundational methods of rabbinic interpretation, appearing throughout the Talmud and midrash (rabbinic interpretive commentary).