It was ruined, though, by the last girl—Number 31, as Barney calls her—as she gets pregnant. Number 31 later gives birth to Barney Stinson's child, Ellie Stinson.
In one of the deleted scenes of Last Forever - Part Two , Barney is at the bar on August 31st of 2019 where he is seen smoking a cigar which then Carl yells at him. Barney tells him that he has had a perfect month but Carl tells him that there are 31 days in August. Barney freaks out and pulls the play that he has one day left to live on a girl, who turns out to be Number 31, the mother of his child. Consequently, this places Ellie's birthday around late May or early June of 2020.