Yes, if by April 18, 2017 you make a contribution to a SEP-IRA for the 2016 tax year, you can amend your 2016 tax return to include it. An IRA contribution is (potentially) a deduction, not a tax credit, so the amount that it reduces your tax bill will be determine by your marginal tax rate on that amount of income. If your marginal tax rate is 25%, you would have to make a SEP contribution of $14,800 to eliminate your tax bill. A SEP contribution of $14,800 would require a net profit from self-employment of something like $79,625.
Yes, if by April 18, 2017 you make a contribution to a SEP-IRA for the 2016 tax year, you can amend your 2016 tax return to include it. An IRA contribution is (potentially) a deduction, not a tax credit, so the amount that it reduces your tax bill will be determine by your marginal tax rate on that amount of income. If your marginal tax rate is 25%, you would have to make a SEP contribution of $14,800 to eliminate your tax bill. A SEP contribution of $14,800 would require a net profit from self-employment of something like $79,625.
Is this true of the 2021 tax year -- that I can file an amended return and make my SEP self-employed contribution by April 18?
Yes it is. But be sure to fund the plan no later than April 18.
Can the missed, late contribution and tax amendment also be made by the extension date of Oct 16?