string strOutput = "19:15:00";
DateTime output = Convert.ToDateTime(strOutput);
Console.WriteLine(String.Format("{0:HH:mm}", output));
Console.WriteLine(String.Format("{0:hh:mm}", output));
Console.WriteLine(output.TimeOfDay);
Console.WriteLine(output.ToShortTimeString());
Console.WriteLine(output.ToLongTimeString());
So, to achieve your answer you need to convert output to DateTime and use the following formatting,
String.Format("{0:HH:mm}", output)
Following references will help you understanding more,
https://msdn.microsoft.com/en-us/library/az4se3k1%28v=vs.110%29.aspx?f=255&MSPPError=-2147217396
String Format for DateTime [C#]