As it sits it is far less flexible than timechart and according the docs your options are: PERIOD (auto | year | month| day | hour | minute | second and that's it. This may (and should!) improve in future releases. Any comment from the devs?
Just bumped into the same thing myself and been poking various plausible options into it...defaults to 5 min buckets if it doesn't recognise it from the above list.