The deadline to file Form 1065 (U.S. Return of Partnership Income) in 2023 is March 15th. However, if the partnership needs more time to file, they can request an extension, in which case the deadline will be extended to September 15th. It is important to note that these deadlines can be subject to change, so it is advisable to check with the Internal Revenue Service (IRS) or consult a tax professional for the most up-to-date information.