Buyers interested in a Tesla have just one week to take delivery before the full federal tax credit gets cut in half—unless Tesla can't get the car delivered until the new year, that is. Then, Tesla says it will make up the difference.

There are some caveats, of course.

CHECK OUT: Tesla (again) says buyers must order today for full tax credit

Tesla CEO Elon Musk tweeted on Saturday that if Tesla committed to make a delivery and a customer made "good faith efforts" to receive the car before the end of the year, Tesla will cover the difference between the new $3,750 half-credit and the full $7,500 credit for 2018.

Musk has twice set deadlines for customers to order new cars to receive the full tax credit, the latest one being the end of November. The latest tweet refers to cars ordered before that point, if Tesla can't deliver them by Dec. 31. He didn't specify what efforts might constitute "good faith" in Tesla's estimation.

Last week, Tesla released all the cars that customers couldn't receive by the end of the year for immediate delivery to new customers. Musk said he would return the deposits of those customers who ordered the cars and couldn't receive them. He also noted that display and test-drive Teslas are available for immediate delivery at discounts off the standard MSRP.

DON'T MISS: Tesla sells 200,000th car, starting phaseout of federal tax credits

The federal tax credits were structured so that after any automaker sells 200,000 plug-in cars, the credits start a wind-down period. At that point, buyers can continue to claim the full $7,500 credit for the remainder of that quarter and for the full quarter following. After that, the credits are cut in half for six months, in half again for another six months. After that, they are eliminated altogether.

Tesla's full-tax-credit marketing campaign covers deliveries until midnight New Year's Eve. The clock is ticking.