keyboard navigation with WAI-ARIA grid widget

Hello

I don't know if you can help me. I'm writing my MSc project on using
WAI-ARIA to make an accessible calendar for people with screen readers;
however, I am finding it very confusing trying to get to grips with the
functionality of the WAI-ARIA grid widget keyboard navigation. In this
document http://www.w3.org/TR/wai-aria-practices/  the information under the
heading "Grid (Simple Data Tables) (widget)" seems to suggest that if you
use the markup for grid, then certain keyboard navigation automatically is
associated with the grid. But all of the examples that I have found for
tables using WAI-ARIA grid markup use javascript to enable keyboard
navigation....would this be in case ARIA is not supported by the browser or
screen reader, but otherwise the navigation should work with just the ARIA
grid markup? Or is it that actually you do need to use javascript to get the
keyboard navigation to work? I have tried using a screen reader with a table
with WAI-ARIA grid markup and didn't find that the keyboard navigation
worked, but I'm not sure if this is because of the screen reader or
incorrect WAI-ARIA markup or that I DO need the javascript.

Thanks for your help,

Jennifer