@이근영 I think the usage of count and offset are correct to fetch older results. I think there should not be a limitation on the results because the field totalEstimatedMatches indicates a large no. of results. The maximum value of count seems to be 100 for news search as per documentation so the offset can be used to paginate. I have tried this scenario and the results seem to only a month old based on date published without any duplicates.
The since parameter seems to support only trending topics to get older results directly though.