I think I see what's going on. It's the way things are worded that confused me. But your math is correct.

All you are doing is taking the total number of days and dividing by the window length.

When you talk of "number of windows", that's where I did what I did because a window can start anywhere, and how many combinations are there of where they can start?

I figured it out visually by hand with pencil and paper on a small sample.

So with that.... correcting my error...

Time Span: 12333 days
Window Size: 3 days
No. Windows: 4111
total quakes: 1261
Probability: .30673801994649

That look better?

Brian