New Post: DateTimeUpDown Custom Format

You seems to use the DateTimePicker to "format" data. You should use standard data API to acheive your need:

DateTime myTime = dtp.Value;
return myTime.ToString("yyyy-MM-dd");

But if that was not the point to your question, you may have more success with this return statement:

DateTimePicker myDTP = new DateTimePicker();
myDTP.Format = DateTimeFormat.Custom;
myDTP.FormatString = "yyyy-MM-dd";
myDTP.Value = dtp.Value;
return myDTP.Text;