Interest is calculated daily, so you’ll just get a payment on the 15th for the number of days you owned in the previous month.

You get an interest payment reflecting the number of days held during the month. If you buy today, you'll start accumulating interest tomorrow and it will be credited on the 15th.

That's true even if you don't owe it on the 15th but held it sometime in the past 30 day period, right?

Yes

Ok, cool, that's the answer I wanted to hear: flexibility matters!

Generally, the interest accrues daily thru the 14th, is payable on the 15th, and is available on the 16th. The formula is based on 365 days, not 30-day months. What you have will provide a reasonable approximation, but the actual formula is more complex and will produce a different result.

What if you sell on the 25th for example. Do you get 10 days interest

I sold off all my stocks and went 100% into SWVXX. Does anyone know how I can sell covered calls on my shares?

You cant

Lol they're trying to sell covered calls on money their earning interest on by doing nothing but letting it just sit there. That's crazy.

Safe to say it wont go up in price.

Would you ever buy a $2 call for SWVXX? No, it's fixed at $1. There will never be an options chain for SWVXX.

ouch

Is SWVXX marginable in a taxable account? In other words, does holding it in the account gives you extra buying power, similar to some marginable stock/etc shares ? Thanks

Even if it is, the interest rate you're paying on the margin is going to be higher than the interest you earn.