There was a patch that prevented reading stats from being shown after you finished a purchased book - see
https://www.mobileread.com/forums/sh....php?p=3932626 - unfortunately it does not seem to work with the current firmware release.
However the trick of resetting the reading time to zero still works. To reset the clock first make sure that WiFi is off then from the Home Screen select "More" > "Settings" > "Energy saving and privacy" go to page 2 and toggle "Automatically share data about the features you use:" On and then Off. Now go back to the Home Screen and check the Reading Stats ("More" > "Activity") and you should find all of the time related statistics reset to zero.