The Passover and the Festival of Unleavened Bread
The Lord said to Moses and Aaron in Egypt
“This month is to be for you the first month, the first month of your year.
What about Rosh Hashana?
What is the Torah saying here?
There are several new years. (What!?) There is a new year for the world, a new year for the Jewish people, a new year for the trees, etc.
Nissan is the first month because it's the new year for the Jewish people, as it's the month the Children of Israel left Egypt. (Throughout the Tanach, whenever the "first month" is mentioned, they are speaking of the month of Nissan.) Notice how the verse says, "This month is to be for you the first month..."
Tishrei is the first month because it's the new year for the world, as it's the month the world was created.
Likewise, Shevat is the first month because it's the new year for the trees, as it's the month all the trees in the Land of Israel begin to bud and bloom.
This is not a strange concept, as even within our daily lives we find there are several new years. New Year's Day may be on January 1, but the new year for school is in late August, early September. And, the new year for taxes begins in April.
You see, there can be many new years.