Fun fact, all US customary units are defined in terms of the metric system. For example, 1 inch is exactly 2.54cm. which means a US mile is 1.609344km. Americans have be using a bastardized version of the metric system since 1959.

https://en.wikipedia.org/wiki/United_States_customary_units

Yeah, you’re not going to be able to get a guaranteed rate on a HYS account. The upside is that the interest rate is high. The downside is that the interest rate will change, usually down, as the market changes, but something is better than nothing. You have to find the balance between maximizing returns and maximizing effort. If you want more that a 3-4% return, you’re likely going to have to start looking into an investment account, but a 3-4% return is still better than the 0.2% return a traditional savings account provides.

All cards calculate interest per day. That’s the standard formula. They only charge you the sum interest at the end of the month when your billing cycle ends.

https://www.nerdwallet.com/article/credit-cards/how-is-credit-card-interest-calculated

This becomes apparent if you’ve ever paid a card off really early, and then still been charged interest even though you’ve had a zero balance one or two weeks before the billing cycle ended.

https://www.bankrate.com/banking/savings/best-high-yield-interests-savings-accounts/

This was the first result that I searched. You can get a HYS with a little to no minimal balance. A lot of them are online banks, but they’re still FDIC insured. There are plenty HYS accounts out there that are usable for everyday people given you have the ability to save.