corporate due date

Are you sure you read the IRS instructions correctly?

Generally, a corporation must file its
income tax return by the 15th day of the
4th month after the end of its tax year. A
new corporation filing a short-period return
must generally file by the 15th day of the
4th month after the short period ends. A
corporation that has dissolved must
generally file by the 15th day of the 4th
month after the date it dissolved.
However, a corporation with a fiscal tax
year ending June 30 must file by the 15th
day of the 3rd month after the end of its
tax year. A corporation with a short tax
year ending anytime in June will be
treated as if the short year ended on June
30, and must file by the 15th day of the 3rd
month after the end of its tax year.
If the due date falls on a Saturday,
Sunday, or legal holiday, the corporation
can file on the next business day.

Sorry, I took you at your word and didn't look at the actual dates correctly.  There is a month difference between the 15th of the 4th month and 4 months and 15 days.