TaxGuyBill
Level 15
You got me curious ... this section of the IRM does not seem to have been updated recently, so I think it has always been April 15th (one year from the due date of the return).

"The third party designation authority expires one year from the due date of the return in question. Extensions of time to file a return do not extend the third party designation period. There are no exceptions or extensions to the expiration date of the third party designation period."
https://www.irs.gov/irm/part21/irm_21-001-003r#idm139697404321456
0 Cheers