ATM the build results tab 'Completed builds' /build/viewBuildHistory!default.action?buildKey=XXX-YYY displays a table where build duration is 'Unknown' if the build took less than a sec:

In AbstractBuildResultSummary.java

public String getDurationDescription()
{

final String DURATION_UNKNOWN = "Unknown";
final double durationInSeconds = getDurationInSeconds();
if (durationInSeconds > 0)

{ return DurationUtils.getPrettyPrint((long) (durationInSeconds * 1000)); }

else

{ return DURATION_UNKNOWN; }

}