According to the IRS instructions, the placed in service date should be the date of the binding contract (8/15/2022). This is per the transition rule:
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Enter 08/15/2022 if you qualify and elect to apply the transition rule discussed below.
Transition rule.
If you purchased, or entered into a written binding contract to purchase, a qualified plug-in electric drive motor vehicle after 2021 and before August 16, 2022, you may elect to treat such vehicle as having been placed in service on August 15, 2022, the day before the enactment date of the Inflation Reduction Act of 2022.
