In response to your new question, your problem is going to be related to the fact that you've switched the date and the month.
Your culture settings probably have the DateTime specified as MM/dd/yyyy, which means that when it sees 15/9/2010, it says there is not month with the number of 15.
Add:
using System.Globalization;
and then change your code to:
CultureInfo provider = CultureInfo.InvariantCulture;
DateTime arrival = DateTime.ParseExact(tb1.Text, "d/M/yyyy", provider);
DateTime departure = DateTime.ParseExact(tb2.Text, "d/M/yyyy",provider);
I'm also curious why you say that you want your format to be in MM/dd/yyyy format and then you use d/M/yyyy. What's up with that? If you had originally stored it as MM/dd/yyyy, you wouldn't have gotten the issue with Parse.