granularity of views filter on date field
1) Double check your field is collecting the correct date attributes.
2) Make sure "Date Views" is enabled under Modules (Requires: Date API, Views, Chaos tools).

If the above are okay then try creating a new date field making sure you're collecting all date attributes. This should rule out any issues there might be with the field. It could also pay to try create a new date field in a different content type and see how views reacts.

If the new date field doesn't work and views still can't filter down to seconds granularity then try a new test view.

Tested with no problems on Drupal 7.18, Date 7.x-2.6 and Views 7.x-3.5.