Yes I did. (I'm the one who put the due date rule information in the board IBox info page because we used to have lengthy due-date debates every time a report was due.) The extension always begins one calendar day after the due date. The due date itself is never considered the first day of the extension. So the fifteen-day extension started on April 2nd.
Look at it this way. If something is due on a Monday, and you are given a one-day extension, then it is due on Tuesday. You don't count the Monday as the first day of the extension, because then it would still be due on Monday. ;0) Similarly if something is due on a Monday and you are given a two-day extension, it is then due on Wednesday. If you counted the Monday as the first day of the extension, it would be due on Tuesday, and your two-day extension would only have given you one extra day.
So to recap, this year being a leap year the annual report due date would have been March 30th. But since that was a Saturday and due dates that fall on non-market days are always adjusted to the next market day, the due date became Monday, April 1st. The extension gave an additional fifteen calendar days beginning on April 2nd, making the due date April 16th.