Date formatting yyyymmdd to yyyy-mm-dd

tdrDate = DateTime.ParseExact(dateString, "yyyyMMdd", null).ToString("yyyy-MM-dd");

You need MM, not mm. mm is for minutes.

It should be:

DateTime.ParseExact(dateString, "yyyyMMdd", null).ToString("yyyy-MM-dd");

Capital 'MM' in the first date format string.

"yyyymmdd" must be "yyyyMMdd".

mm is for minutes.