I got an error .. " Unable to convert '' (from DataStartField column) to DateTime."

What is the actual value you are trying to convert? Are you passing in a nullable datatype?

Steve is right: You should check the values in this column. It looks like one of the values is null.

DayPilot uses the following code to get the DateTime value from the DataStartField column:

            string strStart = DataBinder.GetPropertyValue(dataItem, DataStartField, null);
            if (!DateTime.TryParse(strStart, out start))
            {
                throw new FormatException(String.Format("Unable to convert '{0}' (from DataStartField column) to DateTime.", strStart));
            }