Datetime NowMinus30Days = Datetime.Now;
NowMinus30Days = NowMinus30Days.AddDays(-30.0);
string LogDate = NowMinus30Days.toString("s");

Siehe auch:

• http://www.csharp-examples.net/string-format-datetime/
• https://msdn.microsoft.com/de-de/library/az4se3k1%28v=vs.110%29.aspx?f=255&MSPPError=-2147217396
• https://msdn.microsoft.com/de-de/library/8kb3ddd4(v=vs.110).aspx