The new tax law removes the option to recharacterize a Roth conversion starting in 2018. It was unclear as to how they would treat a Roth conversion done in 2017 but which one might desire to recharacterize in 2018. Per this thread, the IRS is saying you will be allowed to recharacterize 2017 conversions if you wish. The deadline is the normal one, up until when you file your 2017 taxes, possibly including an extension.
Of course with the big bull market looking to continue, there may not be much reason to, but it’s good to know we get one last option before the full force of the new rules kick in.