"Valid through 2021" means until when?

If some validity period, say a promotion, specifies "valid through 2021", does that mean until Dec 31, 2021, or Jan 1, 2021?

This is in the USA, if this differs around the world.

In normal use, in the US, when "through" is used to refer to a chronological range, it implies that the range extends to the end of the day/month/year stated.

"Sale runs through Friday" implies that it goes until the end of business on Friday.

"Coupon valid through 2021" implies that it goes until the close of business on the last business day of 2021. Since Dec 31, 2021 is a Friday, and not a holiday in the US, this would imply the validity runs through the close of business on Dec 31. (But note that many businesses close early on New Year's Eve.)