There are some indications that it might have been spreading possibly as early as September but the coverage I’ve seen was preliminary and researchers were cautious about concluding anything without more comprehensive tests to rule out things like cross-reactivity with other coronaviruses. The generally accepted patient 0 for the outbreak must have been infected in November to be symptomatic in early December but I haven’t seen any credible claims that anyone in China had identified this as a new disease until late December.