Not sure why this was down-voted, but this has been one of my concerns about ever moving to CA in the first place--I wouldn't want to be stalked by this predatory creature for the rest of my life. (And have already experienced this to a lesser degree with a couple of other-coast states.)
Yeah, I was wondering how that'd apply to getting accepted in YC... Maybe you intend to go back home after the 3 months or locate to another city big in tech like Austin or Boston. Wonder if California would try to say you were a resident, however I feel like YC would be more similar logically to how college students are treated since usually you are still a resident of where your parents live, unless you intend to get a job and live where you went to school after graduation.
In that case, if it comes up, I think California would be looking at your living situation: are you in a temporary living situation, like a long term stay hotel, or did you rent an apartment and move your stuff. Did you forward your mail and change your accounts? Are you visiting home, or are people visiting you in California. What did you actually do after the YC period, etc.
If you're living and working in California, there's not a whole lot of difference in taxation between a part-year resident and a non-resident who happens to be working in the state often; it's more of a problem when California considers you a full year resident and you're actually working somewhere else. For residents, during the period of residency, all income is treated as California source income, but for non-residents, California only taxes income that is actually from a California source (basically earned income from working in California, or gains on property in California).
Even if you're considered a resident during YC, you wouldn't be taxed on non-California source income before you moved in, or after you moved out. It's usually not a problem when you legitimately move; it's more of a problem when you keep a house in California, and visit frequently, and still get your hair cut in California, still vote in California, etc... Or in the case that you move overseas --- there's a presumption for US citizens that an overseas move is not a permanent move, and that when you come back, you'll return to the last state that you resided in, and many states with all-source taxation for residents will make a strong suggestion that you're still a resident, until you establish residency in another state.
California allows a domiciled taxpayer to be taxed as a nonresident of the state if they are outside of California for an uninterrupted period of at least 546 days under an employment related contract, unless they have intangible income of greater than $200,000, or the principal reason for their absence is tax avoidance.1 The employment related contract can be in another country or another state. The taxpayer is also allowed to have up to 45 days of presence in the state each tax year before they no longer can claim the safe harbor. A person can start filing as a nonresident on the presumption that their out-of-state assignment will last the required 18 months; but if for some reason their foreign employment terminates early and the 546 day requirement will not be met, any tax years for which the taxpayer treated themselves as a nonresident must be refiled as a resident and all taxes owed for that period must be paid.
