The DateTime represented by the string is not supported in calendar System.Globalization.GregorianCalendar

Problem : You are parsing the Month first using MM/dd/yyyy,
but you are giving date first like this 31/04/2012

Solution : You need to use dd/MM/yyyy format.

Try This:

usinfo.BirthDate = DateTime.ParseExact(txtDOB.Text.ToString(), "dd/MM/yyyy", 
                                                CultureInfo.InvariantCulture);

EDIT: You entered an invalid Date here: 31/04/2012.

The month April does not have 31 days. It has 30 days. So you need to make sure you enter a valid date.