Ex-dividend is the time period between the announcement and payment of a dividend, while the date of record is the day a shareholder must officially own shares to be entitled to the dividend.
The ex-dividend date generally precedes the record date, usually by four business days on the New York Stock Exchange. But a NYSE rule allows the ex-dividend and record dates to be flip-flopped when a dividend is more than 25% of the current stock price. Why? As the ex-dividend date nears, usually a stock's price will rise by the dividend amount, then fall by that much after the date. A big dividend distribution will knock the stock price way down, but by pushing that date off, investors have a few extra days to trade the stock at the higher price.