@Override
    protected int onGetClassId() {
        return classId;
    }
<p>Create a bucket expression with the given width and the given subexpression.</p>

@param arg  The argument for this function.
@param part The part of time to retrieve.
@param gmt  Whether or not to treat time as GMT.
searchlib/src/main/java/com/yahoo/searchlib/expression/TimeStampFunctionNode.java