From the episode:

> **O'Brien:**  "Well, we know one thing.  They arrived before the year 2048."<br>
> **Odo:**  "How can you be sure?"<br>
> **O'Brien:**  "'Cause we were just there...  And it's nothing like the mid-21st century I read about in school.  It's been _changed_..  I mean, Earth history has been through its rough patches, but never that rough."<br>
> **Kira:**  "If we limit our search to dates before 2048, how many possibilities does that leave us with?"<br>
> *(Odo hands PADD to O'Brien)*<br>
> **O'Brien:**  "According to my calculations, three.  But we only have enough chroniton particles for one more try."<br>
> **Kira:**  "Well..  Pick one, chief!  Lets hope you get lucky."<br>
> **O'Brien:**  (examines the PADD for a moment, taps a date, hands it back to Odo) "It's my best guess."<br>
> **Kira:**  "That's good enough for me."<br>

We have a Bajoran, an alien shapeshifter, and Miles O'Brien on-hand.  Kira and Odo have little to no knowledge of Earth's history (at least, pre-Federation (which was 2161, over a hundred years after 2048)).  And just look at O'Brien's face when he's trying to figure it out:

![O'Brien with wide-eyes][1]

Kira put him on the spot (not too unreasonable given the circumstances), and he made his best guess given the available information, since no one else was on-hand to help, and Kira, Odo, and everyone else was relying on him.  His "best guess" _is_ an estimated guess, given limited information.

The other two points in time apparently weren't critical moments, which is why he chose the Bell riots.

  [1]: https://i.sstatic.net/SCZZp.png