this might help:
https://www.treasurydirect.gov/marketable-securities/treasury-bills/
Bills: (under 1 year maturity)
Interest rate
| Fixed at auction. For bills, "interest" is the difference between what you paid and the face value you get when the bill matures. |
Notes (1 year or greater maturity)
Interest rateInterest paid
The rate is fixed at auction. It doesn’t change over the life of the note.
It is never less than 0.125%.
See Results of recent note auctions. |
| Every six months until maturity |
that you are buying these securities via a middle man (the secondary market) doesn't change what you are buying and therefore the calculation of interest.
By the way, buying directly from Treasury Direct is VERY EASY
