You require the answer on this post, but it does not contain the question... 
API description presented here ( https://www.chess.com/news/view/published-data-api ) is not always up to date with respect to the current abilities of API. I guess it will be updated some day or other, and meanwhile I believe that you can understand, what fields "start_time" and "time_class" mean without detailed explanation. 
That's not the point. The documentation is inaccurate and therefore doesn't present the correct information for other users,
As I said, it is not something surprising here. From time to time it is updated by the developers (the last one was made the 22nd of January, 2020), but quite often the documentation does not fully describe the last changes in API...
@Tricky_Dicky We have added "start_time" and "time_class" to relevant endpoint. Thanks for spotting it.
Returned endpoint, https://api.chess.com/pub/club/{ID}/matches, does not conform to specification.
API data includes "start_time": & "time_class":
Not in specification