There are 2 dates on a Letter of Determination, the stamped date, which is the date the IRS gives the determination (or the ruling), and the “Effective Date of Exemption”, this is the date that the org is exempt from taxes and donations can be made that are tax deductible. See the example below.
If you have any concern about these dates you would need to follow up with the IRS. You may contact the Nonprofit Division of the IRS for more information at: (877) 829-5500 (toll-free number). The IRS call center is open 8:00am to 5:00pm Eastern Time.