在按月分组返回的一组数据中，只有初始月份返回的数据不正确

I am trying to return data for the last 24 months as of the end of last month. Only the initial month returns incorrect data, while all the other months are correct. I believe the issue is in the strBeginDate = section of the code. Any ideas what would be causing partially returned data for the initial month? Thank you very much.I am trying to return data for the last 24 mont