I noticed that my taxable income is significantly below the cut-off for the 25% tax rate. I would like to convert part of a Rollover IRA to a Roth IRA so that I can have that contribution only taxed at 15% — I think in the long term this is to my advantage. I discovered this in Feb 2017; is it too late to convert the money — i.e. did this need to be done before Dec 31 2016 or can I do this before April 15, 2017?
No. Conversions are reported for the tax year that the conversion takes place. A 2017 conversion would be reported next year on your 2017 tax return.
The April 18, date is for new IRA contributions, not conversions.