Hey there,
To be frank, I have no certain idea what Kroll checks but here is my take:
1. I doubt they check travel records. My guess is that the focus is more on: Do you work where you say you did? In what capacity? At what salary? Are you really an Olympic ballroom dancer and world-class martial artist who must register your hands as lethal weapons at the airport?
2. Even if they DID check this, AND thought it was important enough to report back to Columbia, I'd be surprised if Columbia cared. Unless you're claiming to have done something amazing that could only have been accomplished in six months and not in five, or you need an alibi for that sixth month in order to evade a criminal charge, who cares?
3. If, in a worst case scenario, Columbia finds out
and they care, they'll probably just contact you for clarification. At which point you can graciously apologize for the error; after all, the number 6 is right next to the number 5 on the keyboard. Typos are to be avoided at all costs, but hey - they happen.
In short, unless there's some part of the story you're not telling us ... be easy. And congratulations on your acceptance!