Why the landing date
The same money is taxed differently before and after
Under the green card test, tax residency begins on the first day you are in the United States as a permanent resident. That year is usually a dual-status year: nonresident before, resident after.
A few exceptions need checking first: if you already spent many days in the U.S. in the landing year, residency may begin earlier; a married couple electing to file jointly as full-year residents brings pre-landing foreign income into the return; and U.S. real estate and U.S. business assets are taxed on sale even for nonresidents. So the first job is to pin down your residency starting date.