Without actually trying it, I'm not sure this is possible. The largest clock prescaler is 16384, so with a 16.5 MHz system clock and using the overflow interrupt, the slowest the ISR could run would be
16,500,000 / 16,384 / 256 = 3.934 Hz
I might not use a web page that calculates ATmega timer values for an ATtiny.
Yes you are correct.
Well, you probably can't get the hardware to call your ISR at one second intervals, but you could set up a counter in your ISR that calculates when a one second interval is, and calls your code accordingly.
I think the standard 328 core uses a FRACT calculation because it uses the standard 256 top and prescaler of 64. That doesn't divide evenly into 16,000,000. It's close to a millisecond, but not exact. So they have to calculate how far off it is every time so there isn't any drift.
If all you want is one second timer, you could set up your timer interrupt with a top of 250 and a prescaler of 8. Then you could count down to 20625 and your code would be called at exact one second intervals.
Thanks! I'll try to implement this and post results.