Only return stories created after this time. Applies to every story type; pair it with resource_subtype=comment_added to get just the comments added since a given moment.
The bound is exclusive but has second granularity: it is truncated to the start of its second before filtering, so any story created within that same second is still returned. A client polling with the created_at of the last story it saw should expect to see that story again, and others alongside it, and should de-duplicate on gid.