The last date to contribute to an IRA for tax year 2016 would be on April 18, 2017. An extension request does not give you an extra six months to contribute to an IRA for 2016. Any contributions after April 18, 2017 would be reported on the tax year 2017 return filed during 2018.
https://www.irs.gov/publications/p590a/ch01.html#en_US_2016_publink1000230424
Contributions must be made by due date. Contributions can be made to your traditional IRA for a year at any time during the year or by the due date for filing your return for that year, not including extensions. For most people, this means that contributions for 2016 must be made by April 18, 2017.
Due date for contributions. Because April 15, 2017 falls on a Saturday, and Emancipation Day, a legal holiday in the District of Columbia is observed on Monday, April 17, 2017, the due date for making contributions for 2016 to your IRA is April 18, 2017, even if you do not live in the District of Columbia.
