Hello SUNIL MALI,
This kind of issue will arise, if your system (server if it is web app/where this app running), Date Time format is different from your coding format, it may like MM/dd/YY, chekc this
Modify that to dd/MM/yyyy as your format is like this 21/10/2013, then you can get this work
then User following
DateTime dt = DateTime.Parse(txtRStartDate.Text);
DateTime dtChanged = dt.AddYears(1);
dtChanged = dtChanged.AddDays(-1);
txtREDate.Text = Convert.ToString(dtChanged);
Thanks
Happy Coding