Calculate age in C#

An easy to understand and simple solution.

// Save today's date.
var today = DateTime.Today;
// Calculate the age.
var age = today.Year - birthdate.Year;
// Go back to the year the person was born in case of a leap year
if (birthdate > today.AddYears(-age)) age--;

This is a strange way to do it, but if you format the date to yyyymmdd and subtract the date of birth from the current date then drop the last 4 digits you’ve got the age 🙂

I don’t know C#, but I believe this will work in any language.

20080814 – 19800703 = 280111
Drop the last 4 digits = 28.

C# Code:

int now = int.Parse(DateTime.Now.ToString("yyyyMMdd"));
int dob = int.Parse(dateOfBirth.ToString("yyyyMMdd"));
int age = (now - dob) / 10000;
</code>
Or alternatively without all the type conversion in the form of an extension method. Error checking omitted:
<code>
public static Int32 GetAge(this DateTime dateOfBirth)
{
    var today = DateTime.Today;

    var a = (today.Year * 100 + today.Month) * 100 + today.Day;
    var b = (dateOfBirth.Year * 100 + dateOfBirth.Month) * 100 + dateOfBirth.Day;

    return (a - b) / 10000;
}

I don’t know how the wrong solution can be accepted.
The correct C# snippet was written by Michael Stum

Here is a test snippet:

    DateTime bDay = new DateTime(2000, 2, 29);
    DateTime now = new DateTime(2009, 2, 28);
    MessageBox.Show(string.Format("Test {0} {1} {2}",
                    CalculateAgeWrong1(bDay, now),     // outputs 9
                    CalculateAgeWrong2(bDay, now),     // outputs 9
                    CalculateAgeCorrect(bDay, now)));  // outputs 8

Here you have the methods:

    public int CalculateAgeWrong1(DateTime birthDate, DateTime now)
    {
        return new DateTime(now.Subtract(birthDate).Ticks).Year - 1;
    }

    public int CalculateAgeWrong2(DateTime birthDate, DateTime now)
    {
        int age = now.Year - birthDate.Year;

        if (now < birthDate.AddYears(age))
            age--;

        return age;
    }

    public int CalculateAgeCorrect(DateTime birthDate, DateTime now)
    {
        int age = now.Year - birthDate.Year;

        if (now.Month < birthDate.Month || (now.Month == birthDate.Month && now.Day < birthDate.Day))
            age--;

        return age;
    }