IMHO, no federal citation, mostly a bank preference. In the past (B.C. as before computers) many of the commercial notes were for 90 days. That was 1/4 of the annual interest [90/360]. It was easier to calculate than counting days and dividing by 365/366. Added benefit a slightly higher rate of return.
For legality, check your state regs.
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Integrity. With it, nothing else matters. Without it, nothing else matters.