Updating complication with Swift 3 and background task

Question

For watchOS 3, Apple suggests updating the complication with WKRefreshBackgroundTask instead of using getNextRequestedUpdateDate.

How can I determine the time between two updates using the new approach?

I would only hack my data requesting (from an url) into getCurrentTimelineEntry and would update the complication, but I think that's not really what Apple would recommend.

A short code example would be a big help.