Oregon - Interest Rate Rounding and Effective Dates

According to Administrative Rule 150-305.220(1) of the State of Oregon Department of Revenue, subtitled "Computation of Interest on Deficiencies and Delinquencies" (https://web.archive.org/web/20040605041253/http://www.dor.state.or.us/IncomeR/305i.html):

(3) Interest periods. An interest period is each full month starting with the interest starting date and ending one day before the corresponding date one month later. Interest will be figured on a daily basis for a fraction of a month. The daily rate is based on a 365-day year.

(5) Decimal places used in computations. In all computations, the interest rate will consist of six decimal places...

The new interest rate, even though effective on the first day of a month, does not apply until the first day of the first interest period that begins after the effective date. In these examples, the first interest period begins on the 16th of the month.

The language in Proposed Administrative Rule 150-305.220(2), subtitled "Interest on Refunds," follows the same rules.