As we don't have the sample dates you are using, I am assuming that the first portion of you code is working because the date you are passing is correct(unintentionally). For example, passing "10/10/2013" is a valid date both in "dd/MM/yyyy" format as well as "MM/dd/yyyy" format. The second part is not executing because passing "15/12/2013" is correct in "dd/MM/yyyy" format but it is incorrect in "MM/dd/yyyy" format. And maybe the culture of your application is "en-US". So there is an error in the dates you are passing. If the textboxes you are using takes dates in "MM/dd/yyyy" format, your code should work fine. But, if your textboxes are taking dates in "dd/MM/yyyy" format, you can convert it in "MM/dd/yyyy" format by using the below code:
string InputDate = YourDateTextBoxId.Text.Trim();
System.Globalization.DateTimeFormatInfo dateInfo = new System.Globalization.DateTimeFormatInfo();
dateInfo.ShortDatePattern = "dd/MM/yyyy";
DateTime OutputDate = Convert.ToDateTime(InputDate, dateInfo);
OutputDate will be the valid date. Try it and tell us if it works.