If you moved your domicile (your main, permanent home) to New York from another state during 2018, you would file as a part-year resident in both states.
To put it another way, if you became a domiciliary resident of NY at any time during the year, you're a part-year resident.
But if you maintained your domicile in another state, and were in NY only temporarily, here is NY's rule:
You're a (full year) NY resident for tax purposes if:
"Your domicile is not New York State but you maintain a permanent place of abode in New York State for more than 11 months of the year and spend 184 days or more in New York State during the tax year." (Any part of a day is a day for this purpose.)
https://www.tax.ny.gov/pit/file/pit_definitions.htm
>If your domicile is in another state and you did NOT meet NY's definition of resident, then you would file in NY as a non-resident for any NY-source income.
If you moved your domicile (your main, permanent home) to New York from another state during 2018, you would file as a part-year resident in both states.
To put it another way, if you became a domiciliary resident of NY at any time during the year, you're a part-year resident.
But if you maintained your domicile in another state, and were in NY only temporarily, here is NY's rule:
You're a (full year) NY resident for tax purposes if:
"Your domicile is not New York State but you maintain a permanent place of abode in New York State for more than 11 months of the year and spend 184 days or more in New York State during the tax year." (Any part of a day is a day for this purpose.)
https://www.tax.ny.gov/pit/file/pit_definitions.htm
>If your domicile is in another state and you did NOT meet NY's definition of resident, then you would file in NY as a non-resident for any NY-source income.