Select Web Service Template and click on add button. then after that the Solution Explorer look like as follows:


Then open the Webservice.cs class and write the following method followed by [WebMethod] attribute as in:   

In the code above I have declared a one integer method named CalculateAge with the three parameters day, month and year for accepting day, month and year from the user.Then after that I created an object of date time and passed the those variables that I get from the users. I declared another variable in the method that is agetodays to store the number of days remaining from the user's input date to the current date and finally I return that variable.

The webservice.cs file will then look as in the following: 

   [WebMethod]
  public int converttodaysweb(int day, int month, int year)
        DateTime dt =new DateTime(year,month,day);
        int datetodays=DateTime.Now.Subtract(dt).Days;
       return datetodays;   
    }   
}
Now run the application that look like as follows..



Now in the above we see our method that we are created in the webservice.cs file, so click on that method and provide input values and click on the "invoke" link as in:


The output will be as follows: