It must have been that they somehow knew on the issuance date that the stock price was going to go up, but he said they couldn't possibly know that in these general circumstances.
When companies declare a dividend, they choose the amount per share, the date the stock will go ex-dividend, and the date payment will be made to shareholders of record.
If you do wish to account for this, enter the value of the dividend payment with a negative sign on the date a stock goes ex-dividend, and add the number back in when the dividend is received.
