Day 2: 5:00 A.M.-6:00 A.M. - S2-E22
Other mistake: In the beginning of the episode when the show is starting and the credits are rolling, instead of Billy Burke the credit is shown as Billy Gurke.
Day 2: 11:00 P.M.-12:00 Midnight - S2-E16
Other mistake: In German Television the clock at the end switches from 23:59:59 to 23:00:00
Day 2: 7:00 A.M.-8:00 A.M. - S2-E24
Other mistake: In the shootout at the stadium, several times when Jack fires a gun the sound of the gun firing is heard but it is obvious that the gun did NOT fire. Most noticeable when Jack fires the third shot at the man behind the wall and when he is struggling for the gun with the last guy who he beats up.
Day 2: 6:00 A.M.-7:00 A.M. - S2-E23
Other mistake: When President Palmer is at District Headquarters, he addresses one of the workers as "Heather," but the character is credited as "Linda."
Answer: According to Jack in season 5, Nina was "deep cover" before the pair ever met, so it's safe to assume she was recruited before she joined CTU. As for what her primary mission was, who knows? Perhaps it was to gather sensitive intel and pass it on to her superiors. Given that she worked for an intelligence agency, it's not a stretch. Or perhaps we may find out in season seven.