The filing deadline closes most planning doors, but not all of them. A short list of moves that remain available late in the cycle.
By the time the return is being prepared, most of the year's planning is already fixed. A handful of levers, though, stay open right up to the filing date.
Retirement plan contributions, entity elections, and the treatment of certain deductions can still change the number owed. Each has its own cut-off, and they are not all the same date.
The wider point is that a late-cycle scramble is a symptom. Owners who plan quarterly rarely need the playbook at all.