I worked one day in New York state (NYC) yet my W2 allocates almost all of my yearly wages in Box 16 to NY state. This seems wrong. How do I correct this allocation?

NYC is in NY state.

You will need to contact your employer for a corrected W-2 to show the proper amount of income allocated to NY sources.

Please note that if you have only worked one day in NYC, then you would probably not need to file an NY nonresident state income tax return (based on the first bullet point below).

According to the NY Department of Revenue -

If you are a New York State nonresident you must file Form IT-203, Nonresident and Part-Year Resident Income Tax Return, if you meet any of the following conditions:

• You are a nonresident with New York source income and your New York adjusted gross income Federal amount column (Form IT-203, line 31) exceeds your New York standard deduction.
   
• You want to claim a refund of any New York State, New York City, or Yonkers income taxes withheld from your pay.
   
• You want to claim any of the refundable or carryover credits available. 
   
• You had a net operating loss for New York State personal income tax purposes for the tax year, without having a similar net operating loss for federal income tax purposes.