DateTime.AddDays() Method in C#

This method is used to return a new DateTime that adds the specified number of days to the value of this instance.

Syntax:

public DateTime AddDays (double value);

Here, the value is the number of whole and fractional days. The value parameter can be negative or positive.

Return Value: This method returns an object whose value is the sum of the date and time represented by this instance and the number of days represented by value.

Exception: This method will give ArgumentOutOfRangeException if the resulting DateTime is less than MinValue or greater than MaxValue.

Below programs illustrate the use of DateTime.AddDays(Double) Method:

Example 1:

// C# program to demonstrate the
// DateTime.AddDays(Double) Method
using System;
using System.Globalization;

class GFG {

    // Main Method
    public static void Main()
    {
        try {

            // creating object of DateTime
            DateTime date1 = new DateTime(2010, 1, 1,
                                           8, 0, 15);

            // adding the 36 days
            // using AddDays() method;
            DateTime date2 = date1.AddDays(36);

            // Display the date1
            System.Console.WriteLine("DateTime before "+
                      "operation: {0:y} {0:dd}", date1);

            // Display the date2
            System.Console.WriteLine("\nDateTime after"+
                     " operation: {0:y} {0:dd}", date2);
        }

        catch (ArgumentOutOfRangeException e)
        {
            Console.Write("Exception Thrown: ");
            Console.Write("{0}", e.GetType(), e.Message);
        }
    }
}

Output:

DateTime before operation: 2010 January 01

DateTime after operation: 2010 February 06

Example 2: For ArgumentOutOfRangeException

// C# program to demonstrate the
// DateTime.AddDays(Double) Method
using System;
using System.Globalization;

class GFG {

    // Main Method
    public static void Main()
    {
        try {

            // creating object of DateTime and
            // initialize with MinValue
            DateTime date1 = DateTime.MaxValue;

            // Display the date1
            Console.WriteLine("DateTime before "+
               "operation: {0:y} {0:dd}", date1);

            // adding the TimeSpan of 36 days
            // using Add() method;
            System.DateTime date2 = date1.AddDays(36);

            // Display the date2
            System.Console.WriteLine("\nDateTime after "+
                       "operation: {0:y} {0:dd}", date2);
        }

        catch (ArgumentOutOfRangeException e)
        {
            Console.WriteLine("\nThe resulting DateTime is "+
                       "greater than the DateTime.MaxValue ");

            Console.Write("Exception Thrown: ");
            Console.Write("{0}", e.GetType(), e.Message);
        }
    }
}

Output:

DateTime before operation: 9999 December 31

The resulting DateTime is greater than the DateTime.MaxValue 
Exception Thrown: System.ArgumentOutOfRangeException

Note:

• This method does not change the value of this DateTime. Instead, it returns a new DateTime whose value is the result of this operation.
• The fractional part of the value is the fractional part of a minute. For example, 7.5 is equivalent to 7 minutes, 30 seconds, 0 milliseconds, and 0 ticks.
• The value parameter is rounded to the nearest millisecond.

Reference:

• https://docs.microsoft.com/en-us/dotnet/api/system.datetime.adddays?view=netframework-4.7.2