There is a lot of confusion between timing for IRA contributions and back-door Roth
conversions. Roth IRA conversions are on a calendar year basis. You can convert from a TIRA (Traditional IRA) any time you have a balance in an IRA. No matter what day you convert, it will be counted on that calendar year’s tax return. On the other hand, IRA contributions can be made until April 15 for the previous tax year. So, for example, if you contribute to a TIRA on 2/5/17 and convert to a Roth IRA (back-door maneuver), your TIRA contribution could be claimed on either your 2016 or 2017 income tax return. However, the back-door Roth conversion would be reported on your 2017 income tax return.
What is a backdoor Roth IRA, anyway?