Yes, but this will cause your underpayment penalty to be artificially higher than it should be. That is, if you made your second quarter payment in June as you would in a normal year, but report it as part of your third quarter payment made in September, your penalty will be calculated as if you made the second payment three months later than you actually did. Since the underpayment penalty is calculated based on the dates estimated payments are made, the only way to get an accurate result is to get credit for payments made the day they were made. As I read line 19, the IRS will give you credit for all payments made prior to July 15 as if on time and therefore a penalty will only accrue from July 15 forward if your accrued payment in column (a) lags that of Line 18.
