Even if you were a resident of New York for 1 day of the tax year, NY uses your entire AGI to calculate your tax on NY income.
It isn't, however, as bad as it appears. They calculate what your New York tax would have been if you earned everything in NY, but then prorate that tax based on how much of your income was earned while a NY resident.
For example, if the calculated NY tax on your entire 2015 income was $10,000 ,but only 30% was earned as a NY resident, your NY tax would be 30% of $10,000 = $3,000.
Why do they do this? So they can tax your NY income at the highest possible tax rate, based on your entire AGI.