If a calendar-year S corporation does not request an automatic
six-month extension of time to file its income tax return,
the return is due by

(b) The requirement is to determine the due date of a
calendar-year S corporation’s tax return. An S corporation must
file its federal income tax return (Form 1120-S) by the 15th day
of the third month following the close of its taxable year. Thus, a
calendar-year S corporation must file its tax return by March 15,
if an automatic six-month extension of time is not requested.