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