"The vehicle must be acquired for use or lease and not for resale. Additionally, the original use of the vehicle must commence with the taxpayer and the vehicle must be used predominantly in the United States. For purposes of the 30D credit, a vehicle is not considered acquired prior to the time when title to the vehicle passes to the taxpayer under state law."
Basically what counts is how your individual state determines when you take title. In California, that date is the date in which Tesla mails the forms to the DMV.
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Sorry, here's the link: