January something 2019 for start

Due date with extension for the 2018 return plus 3 years for stop.

Is efiling open now for a  2018 return?

Yes. Why didn't you just stop with that easy question.
Every year you can file the current return year (2019 in 2020) and two (2) prior years (2017 - 2018) until IRS closes down sometime in December.