As tagteam indicates, an estate's tax year is not permitted to be greater than 12 months. March 20, 2023 through March 31, 2024 would be greater than 12 months. An estate that has its first tax year begin on March 20, 2023 can have its end date no later than February 29, 2024 (and must end on the last day of a month).
The deadline to file the Form 1041 for an estate tax year ending on February 29, 2024 is June 17, 2024. (June 15, 2024 is a Saturday.)
