How are CD Interests computed when bought on the market?

External question — Personal Finance Stack Exchange Author: Alexis Wilke Original post: https://money.stackexchange.com/questions/168050 License: CC BY-SA 4.0 — https://creativecommons.org/licenses/by-sa/4.0/ Adaptation: HTML converted to plain text; contact email addresses removed. I bought a CD that matured and got the principal back and one interest payment. Here are the transactions: Date Description Quantity Price Amount 08.26.2025 Purchase 2,000 $100 -$20,000.00 12.08.2025 Interests $209.42 12.08.2025 Matured -2,000 $100 $20,000.00 The interest was set at 4.2% and it was a 3 months CD. I bought it early to get it at the $100/share (although I know at times you can get it at a smaller price, but you need to be lucky and may have to pay interests for a few days... not worth my while). So, I thought the interests would be: $20,000.00 x 4.2% / 4 = $210.00 But as we can see above, it was only $209.42. I'm just wondering where did the $0.58 cents difference go? From my statement, there were no fees whatsoever. I don't think that's relevant, but I have a trading account with Charles Schwab in the US.

External answer — Personal Finance Stack Exchange Author: 0xFEE1DEAD Original post: https://money.stackexchange.com/a/168051 License: CC BY-SA 4.0 — https://creativecommons.org/licenses/by-sa/4.0/ Adaptation: HTML converted to plain text; contact email addresses removed. The interest is based on the settlement date/effective date, which was likely 9/8/2025. There's no "buying it early" to get a better yield, it's simply issued at par. $20,000 * 4.2% * 91/365 = $209.42