in the line
Duration = Date.FromOADate(Time_OutTextBox.Text - Time_InTextBox.Text)
you are substracting two strings (not DateTime objects yet!) and then call the function Date.FromOADate to convert a double to a DateTime..
If you want to do DateTime arithmetics you should first convert the strings to a DateTime:
Duration = Convert.ToDateTime(Time_OutTextBox) - Convert.ToDateTime(Time_InTextBox)
Hope this helps...
By the way, this looks like vb
.net and not vb6? Then, if my answer isn't exactly what you were looking after, you could be better off posting your question in one of the .NET forums to reach those experts..