How does the date of a certificate of service for the first state habeas petition be determined based on the prison mailbox rule?

^2. The filing date of documents submitted when petitioner was proceeding pro se is determined based on the prison mailbox rule. Houston v. Lack, 487 U.S. 266, 276 (1988) (documents are considered filed at the time prisoner delivers them to prison authorities for mailing). However, the certificate of service for the first state habeas petition states that it was mailed on May 30, 2017, even though it was received by the court on May 24, 2017. Lod. Doc. 4 at 29. The date the petition was received by the court will therefore be considered the date of filing.