try this:
IFormatProvider theCultureInfo = new System.Globalization.CultureInfo("en-GB", true);
DateTime from_date = DateTime.ParseExact(txt_from_date.Text, "dd/MM/yyyy", theCultureInfo);
DateTime to = DateTime.ParseExact(txt_to_date.Text, "dd/MM/yyyy", theCultureInfo);
txt_total_days.Text = ((to - from_date).TotalDays).ToString();
if u any problm, please post it.