webMethods is a company, not a product. There is no such thing as “webMethods 6.1” much like there is no such thing as “Microsoft 2000.”
As you undoubtedly know, but for the sake of readers that may not, the BrokerDate class is part of the webMethods Broker product (COM.activesw.api.client.BrokerDate). If you have code that is interacting directly with the Broker (e.g. a Broker client application written in Java) then you can continue to use BrokerDate.
If you are converting a previous Enterprise Server 4.x/5.x solution to use IS 6.x facilities then you’ll want to review the WmPublic package within Integration Server. Look for the pub.date folder and review the services there. When using IS to do the equivalent of BI/VI/EI work all of the Broker APIs get pushed behind the scenes.
There are migration guides available on Advantage. You may find them helpful.