valFn = function(v) {
            if (v == null) return true;
            return __isDuration(v);
        }
Returns a ISO 8601 duration string  Validator.
    @example
        // Assume em1 is a preexisting EntityManager.
        var eventType = em1.metadataStore.getEntityType("Event");
        var elapsedTimeProperty - eventType.getProperty("ElapsedTime");
        // Validates that the value of the ElapsedTime property on Customer is a duration.
        elapsedTimeProperty.validators.push(Validator.duration());
    @method duration
    @static
    @return {Validator} A new Validator