mariner, we know exactly what year certain events occurred in! We’re currently in 5769, right? If you look in the chumash, you can figure out that Avraham was born in the year 1948. Yitzchak was born 100 years later, so that’s 2048. We know that the Jews left Mitzrayim 400 years after Yitzchak was born, so that’s the year 2448. We’re not missing years there.
Now, since we’re going to make Birchas Hachama this year, in 5769 (an odd numbered year), it can’t have been done in 2448 (an even numbered year), since it must always have been in an odd numbered year.