Interest starts right away, but won’t show in TreasuryDirect until after three months have passed, since TreasuryDirect shows the current redemption value, including the most recent 3 month interest penalty that applies until 5 years old.

So if you buy in January 2023 and sell in January 2024, you get the interest for January 2023 through September 2023, with October through December being forfeited.