As I read it, the Act says:
- the previous credit is extended to a new end-date for each manufacturer
- a new "end-of-credit" timer for Tesla begins counting at 200,000 on the day the Act passes
- if you buy after the day the Act passes (transition period), the credit is reduced to $7k
- if you buy after 600,000, phaseout applies
Most of the language is describing the new phase-out period. It does not say boo about the period between when the previous Act expired, and the day this Act takes effect. In fact, one possible interpretation is that this Act is simply _extending_ the previous Act, and is not a new credit; so the previous credit was extended until 600,000, so all purchases since the previous act expired are retroactively credited.
My guess is that this is an unintentional omission. The Act needs to be clarified.
My feeling is that US EV sales are still so low that giving the credit for all of 2021 is (sadly) not a significant federal cost, and would be cleaner tax policy. Jan-Feb are slow sales months anyways. Specifying a beginning quarter (like Q2 2021) would work also. But specifying a later date (like 1/1/2022, or the date of passage of the Act) will hold-back EV sales during the April-June peak sales season, which is the exact opposite of the goal of the Act. I'd encourage everyone to call or email their US Rep and ask them to contact their peers in Ways & Means, and request a clear start date (preferably soon, to avoid damaging 2021 EV sales).