26 USC 6622 - Interest compounded daily

(a) General rule 
In computing the amount of any interest required to be paid under this title or sections 1961 (c)(1) or 2411 of title 28, United States Code, by the Secretary or by the taxpayer, or any other amount determined by reference to such amount of interest, such interest and such amount shall be compounded daily.
(b) Exception for penalty for failure to file estimated tax 
Subsection (a) shall not apply for purposes of computing the amount of any addition to tax under section 6654 or 6655.