Set the year in a DateTime object

Question

How would I go about changing only the year in a datetime object?

I need to call a number of dates from excel, and if the dates I call are before todays date, set the year to 2099.

e.g. date that is called: 15/11/2010 needs to be set to 15/11/2099

Is this doable? or would it be best to change the whole date?

Answer 1

You just need to create a new DateTime since DateTime is an immutable struct:

DateTime newDt = new DateTime(2099, dt.Month, dt.Day);

If you want to handle the leapyear issue(source date is in leapyear but target year not), you could use following extension method:

public static DateTime ChangeYear(this DateTime dt, int newYear)
{
    return dt.AddYears(newYear - dt.Year);
}

This will retain even the Ticks and the Kind, so it's better than using the DateTime constructor. It will return 02/28/2099 if the orginal DateTime was 02/29/2016(leap year).

You can use it for example in this way:

DateTime dt = new DateTime(2016, 2, 29);
DateTime newDt = dt.ChangeYear(2099); 

Answer 2

DateTime is immutable type. You cannot change year or any other part of it. You should create new DateTime instance with new year. There is no other option.

var date1 = DateTime.Now;
var date2 = new DateTime(2099, date1.Month, date1.Day);

---

Though you can write an extension if you need to do it often:

public static DateTime JumpToYear(this DateTime date, int year)
{
    // you can also provide Hour, Minute, Second and Millisecond
    return new DateTime(year, date.Month, date.Day);
}

Usage:

var date2 = date1.JumpToYear(2099);

Note: February 29 date can become invalid when you change year.