31 U.S.C. § 3103

(a) With the approval of the President, the Secretary of the Treasury may borrow on the credit of the United States Government amounts necessary for expenditures authorized by law and may issue notes of the Government for the amounts borrowed and may buy, redeem, and make refunds under section 3111 of this title . The Secretary may prescribe conditions under section 3121 of this title . Notwithstanding section 3121(a)(5) of this title , the payment date of each series of notes issued shall be at least one year but not more than 10 years from the date of issue. (b) The Government may redeem any part of a series of notes before maturity by giving at least 4 months’ notice but not more than one year’s notice. (c) The holder of a note of one series issued under this section with the same issue date as another series of notes issued under this section may convert, at par value, a note of the holder for a note of the other series.