How to add date in C# project.

That would be because you might be applying the + operator to the dateTime object. Well the + operator has been overloaded for using with DateTime objects, but that is to add two different dateTime objects, like this one

var date1 = new DateTime();
var date2 = new DateTime();

var newDate = date1 + date2; // valid code

The operator overloading can be viewed and learnt more about here on MSDN[^].

What you need is a function of the DateTime object, known as AddDays(), which accepts double value for you to add the days to your current DateTime instance.

For example the following code,

var dateTime = new DateTime();
dateTime.AddDays(1); // adds one day to the current instance.

For more, please go through this MSDN link[^].

DateTime currentdate = Convert.ToDateTime(DateTime.Now.ToShortDateString());
DateTime newdate;
newdate = currentdate.AddDays(1);
TextBox1.Text = newdate.ToString("dd/MM/yyyy");