may make an adjustment without interest with respect to such taxes provided that all such taxes for the time period specified in paragraph (a)(2) (except for amounts that are subsequently paid pursuant to an interest-free adjustment under § 31.6205-1 ) are paid on or before February 2, 1987.
Example. A State or local government employer should have withheld and paid $100 dollars in hospital insurance taxes for the quarter beginning April 1, 1986, and ending June 30, 1986. The due date for the return and payment for that period is July 31, 1986. If the employer made the payment by February 2, 1987, then, under section 6601, interest is not assessable with respect to the underpayment of the hospital insurance taxes. If the employer did not make the payment by February 2, 1987, the interest is assessable for the period from July 31, 1986, until the time of payment.
26 C.F.R. §31.6205-2