I moved to California from the UK mid FY year 2023. I still have not completed 183 days as of Dec 31st 2023 from my date of Arrival. Am I a tax resident? Can I consider date of arrival as the day I became resident?

No.  You are not a resident for federal purposes until you complete the 183 days, so if you did not meet it in 2023, then you would not be a resident for tax purposes.  You would need to file a 1040NR as a non-resident alien for 2023.  You can use Sprintax.com to file the 1040NR as TurboTax does not support that form.

In order to file using TurboTax, if you are not a US Citizen, then you would need to either meet the Substantial Presence Test or the Green card Test.

CA does not have the 183 day rule, which means you could technically be a resident of CA but not for federal purposes.  However, since you still need to file the 1040NR if you do not meet the residency requirements for federal, you would not be able to file your state return through TurboTax as the federal return is required to file the state return.  

@atulhari (edited 4/22/2024 @9:23AMPST)