1. Roth conversion deadline is the same as ROth IRA contribution deadline, 4/15/2019 for 2018 conversion, and 4/15/2020 for 2019 conversion.
2. However, you can make 2019 Roth conversion any time between 1/1/2019 to 4/15/2020.
So if you make a ROTH conversion today, 3/15/2019, you can choose to pay tax in your 2018 tax return, or pay tax in your 2019 tax return.
However, you might owe penalty if you owe too much in your 2019 tax return (less than 90%) if you don't prepay enough tax for 2019 Roth conversion.
