If you file after 27 months, your exemption will only be effective from the application's postmark date.
You can file for an extension of the 27 month deadline by attaching to your 1023 a statement providing the reasons why you failed to complete the 1023 application process within the 27-month period after your incorporation. You can find the acceptable reasons in the instructions for 1023. They include bad advice and information from a lawyer, accountant, or IRS employee.
NOTE: Three groups are not required to file Form 1023: churches, public charities that do not have gross receipts of more than $5000 in each year, and subordinate organizations exempt under a group exemption letter.
Note: This article is not intended to be legal advice. It is advised that you consult your own attorney in regard to your specific situation.