The final tax return filing date for corporations ending on 12/31/07 is 9/15/08. Partnerships, including LLCs filing as partnerships, gets a little longer with a filing date of 10/15/08. The dates are changing next year, though.
Next year partnerships will be subject to the same earlier date as corporations – 9/15. This actually does make a lot of sense because currently an LLC filing as a partnership could legally extend a filing date to 10/15, but leave the member absolutely no time to get his or her tax return done.
If you have a corporation with a different than calendar year end, your first filing date is 2 1/2 months after the year end. You can then make one extension of 6 months.