I was going to go with "up until whatever time you go to bed at night".  But I am a little confused as to why there would be different deadlines if an extension was filed back in 2018.  Wouldn't they have until e-filing shuts down sometime around November of next year?

Yes. I was thinking in terms of when the return becomes a dead return not efiling! Those deadlines would have been the same dates in 2021.

Stop trying to confuse me :stuck_out_tongue_winking_eye:

I may have tried but I didn't succeed!