The simple answer to this is to apply AddYears as shown below because this is the only native method to add years to the 29th of Feb. of leap years and obtain the correct result of the 28th of Feb. for common years.

Some feel that 1th of Mar. is the birthday of leaplings but neither .Net nor any official rule supports this, nor does common logic explain why some born in February should have 75% of their birthdays in another month.

Further, an Age method lends itself to be added as an extension to DateTime . By this you can obtain the age in the simplest possible way:

List item

int age = birthDate.Age();

public static class DateTimeExtensions { /// <summary> /// Calculates the age in years of the current System.DateTime object today. /// </summary> /// <param name="birthDate">The date of birth</param> /// <returns>Age in years today. 0 is returned for a future date of birth.</returns> public static int Age(this DateTime birthDate) { return Age(birthDate, DateTime.Today); } /// <summary> /// Calculates the age in years of the current System.DateTime object on a later date. /// </summary> /// <param name="birthDate">The date of birth</param> /// <param name="laterDate">The date on which to calculate the age.</param> /// <returns>Age in years on a later day. 0 is returned as minimum.</returns> public static int Age(this DateTime birthDate, DateTime laterDate) { int age; age = laterDate.Year - birthDate.Year; if (age > 0) { age -= Convert.ToInt32(laterDate.Date < birthDate.Date.AddYears(age)); } else { age = 0; } return age; } }

Now, run this test:

class Program { static void Main(string[] args) { RunTest(); } private static void RunTest() { DateTime birthDate = new DateTime(2000, 2, 28); DateTime laterDate = new DateTime(2011, 2, 27); string iso = "yyyy-MM-dd"; for (int i = 0; i < 3; i++) { for (int j = 0; j < 3; j++) { Console.WriteLine("Birth date: " + birthDate.AddDays(i).ToString(iso) + " Later date: " + laterDate.AddDays(j).ToString(iso) + " Age: " + birthDate.AddDays(i).Age(laterDate.AddDays(j)).ToString()); } } Console.ReadKey(); } }

The critical date example is this:

Birth date: 2000-02-29 Later date: 2011-02-28 Age: 11

Output:

{ Birth date: 2000-02-28 Later date: 2011-02-27 Age: 10 Birth date: 2000-02-28 Later date: 2011-02-28 Age: 11 Birth date: 2000-02-28 Later date: 2011-03-01 Age: 11 Birth date: 2000-02-29 Later date: 2011-02-27 Age: 10 Birth date: 2000-02-29 Later date: 2011-02-28 Age: 11 Birth date: 2000-02-29 Later date: 2011-03-01 Age: 11 Birth date: 2000-03-01 Later date: 2011-02-27 Age: 10 Birth date: 2000-03-01 Later date: 2011-02-28 Age: 10 Birth date: 2000-03-01 Later date: 2011-03-01 Age: 11 }

And for the later date 2012-02-28: