I suspect your problem might actually be in your theme. There might be a class definition that's overruling the module's definitions and throwing the layout off.
If you switch your page to Garland and the problem goes away, then you'll know you need to look in your theme to find the offending element. Firebug and similar tools will help you find that quickly.
The default CSS in that whole suite of Date-related modules is pretty robust, in my experience. I've thrown them into some pretty hairy page layouts and they've always performed admirably, so the module's CSS is probably the last place I'd look for a problem.