Corrected entry: Kirk and Picard are in the Nexus. They can go to any point in time, but they both choose to go to the one point in time where the difficulty and danger are at the maximum and the chance of success is at the minimum.
briggs
3rd Aug 2019
Correction: They are also sworn to obey the temporal prime directive, and avoid altering the timeline. They chose a time where they had the potential for success without changing the events of the past significantly.