mnlg:Yes. I seem to recall reading somewhere that the -us verb tense does not really distinguish time. Thus, "Mi devus fari tion" would mean both "I should do that" and "I should have done that" (more skilled speakers are welcome to correct me).

I would never claim to be a more experienced speaker than mnlg, but I also learned that "devus" is technically not time-related, and therefore means both "should" and "should have". I also use the "devintus" form, it flows better for me, and unless I'm feeling very formal and flowery, I don't want to say "estus devinta" every time.