Secure 2.0-Roth catchup

For the Roth catchup provision for off calendar plans,  is the $145,000 look back comp based on plan year or calendar year? Also since this is starting in tax year 2024, does this provision start earlier for off calendars.   For example, a 4/1/2023-3/31/2024, would the Roth requirement start with contributions made in calendar year 2024?  

Section 603, Elective deferrals generally limited to regular contribution limit. Under current law, catch-up contributions to a qualified retirement plan can be made on a pre-tax or Roth basis (if permitted by the plan sponsor). Section 603 provides all catch-up contributions to qualified retirement plans are subject to Roth tax treatment, effective for taxable years beginning after December 31, 2023. An exception is provided for employees with compensation of $145,000 or less (indexed).