Some folks won't need to be told, for everyone else - the reason we use a time window around the news event is because information is not evenly distributed. Some market players get it sooner than the rest of us. When they take a position, it shows up in price action, so we watch price prior to the release, generally -3hrs of the release.
Robert Schiller calls this effect an information cascade, Hank Pruden compares trends to the fashion cycle. Both valid thinking paradigms regarding news event related intraday or swing trends and longer term trends as well.