Time in zones for custom zones is live now. You need to re-analyse existing activities. When you enable a set of custom zones in sport settings it will be added to one or more of the activity detail pages (power, hr or pace depending on the stream or all of them).
You can also add/remove and move time in custom zones using the activity charts button:
I am going to add a moving average option to the custom zones setup. The existing time in power zones are computed using a 5 second moving average of power.