Access to Time spent in PowerZone Zx and Time spent in HF Zone x from Data Field

I quite new to Garmin programming and I try to implement a nice datafield or widget to graphically show the time spent in the various Power Zones.

I see there are (native?) datafields HR Z1, HR Z2, ... and Power Z1, Power Z2 and so on.

Can I access these values from a custom datafield?  

Or is there a way to get these values via the API ?