In order to take a deduction for a specific year, a solo 401k plan must be established and adopted by the last day of that tax year. For a calendar tax year, that date is December 31st.
For contributions, you generally can apply them to the prior year if all of the following criteria are met:
- They are made by the due date of your tax return for the prior year (which includes extensions).
- The plan treated the contributions as though it had received them on the last day of the previous year.
- You do one of the following: (1) specify in writing to the plan administrator (or trustee) that the contributions are intended to be applied to the prior year; or (2) you deduct the contributions on your tax return for the prior year.