Deadline (9)
(Other definitions for ultimatum that I've seen before include "Final warning" , "Final statement of actions required of another" , "A final demand - or else" , "Threat" , "Final offer" .)
'deadline' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for time limit that I've seen before include "Point before which something must be achieved" , "Deadline" , "within which task must be done" , "Prescribed term" , "maximum duration" .)
(Other definitions for et tu brute that I've seen before include "Shakespeare's version of Julius Caesar's last words, in Latin" , "Caesar's reported words on being stabbed" , "Julius Caesar's supposed last words" , "According to the Bard, Caesar's final words" , "Julius Caesar's last words to unexpected assassin" .)