34 Tex. Admin. Code § 25.34
An employee subject to the 90-day waiting period in effect from September 1, 2003, through August 31, 2005, may be eligible to receive a year of TRS service credit if the employee was employed in a TRS-covered position and participated as a contributing member of TRS for the amount of time in a school year required by this title, including § 25.1 of this title (relating to Full-Time Service) and § 25.131 of this title (relating to Required Service). Employment service prior to the date on which a person is eligible for TRS membership may not be used to meet the minimum requirements for service creditable in a school year unless a member purchases it in accordance with applicable requirements. A member may not establish a membership start date earlier than the end of the 90-day waiting period by purchasing the service credit described in this section.
34 Tex. Admin. Code § 25.34