private int GetYearDiff(DateTime start, DateTime end)
{
    int diff = end.Year - start.Year;
    if (end.DayOfYear < start.DayOfYear) { diff -= 1; }
    return diff;
}
[Fact]
public void GetYearDiff_WhenCalls_ShouldReturnCorrectYearDiff()
{
    //arrange
    var now = DateTime.Now;
    //act
    //assert
    Assert.Equal(24, GetYearDiff(new DateTime(1992, 7, 9), now)); // passed
    Assert.Equal(24, GetYearDiff(new DateTime(1992, now.Month, now.Day), now)); // passed
    Assert.Equal(23, GetYearDiff(new DateTime(1992, 12, 9), now)); // passed
}

我们是否需要考虑小于1岁的人？作为中国文化，我们将小婴儿的年龄描述为2个月或4周。

下面是我的实现，它不像我想象的那么简单，尤其是处理2/28这样的日期。

public static string HowOld(DateTime birthday, DateTime now)
{
    if (now < birthday)
        throw new ArgumentOutOfRangeException("birthday must be less than now.");

    TimeSpan diff = now - birthday;
    int diffDays = (int)diff.TotalDays;

    if (diffDays > 7)//year, month and week
    {
        int age = now.Year - birthday.Year;

        if (birthday > now.AddYears(-age))
            age--;

        if (age > 0)
        {
            return age + (age > 1 ? " years" : " year");
        }
        else
        {// month and week
            DateTime d = birthday;
            int diffMonth = 1;

            while (d.AddMonths(diffMonth) <= now)
            {
                diffMonth++;
            }

            age = diffMonth-1;

            if (age == 1 && d.Day > now.Day)
                age--;

            if (age > 0)
            {
                return age + (age > 1 ? " months" : " month");
            }
            else
            {
                age = diffDays / 7;
                return age + (age > 1 ? " weeks" : " week");
            }
        }
    }
    else if (diffDays > 0)
    {
        int age = diffDays;
        return age + (age > 1 ? " days" : " day");
    }
    else
    {
        int age = diffDays;
        return "just born";
    }
}

此实现已通过以下测试用例。

[TestMethod]
public void TestAge()
{
    string age = HowOld(new DateTime(2011, 1, 1), new DateTime(2012, 11, 30));
    Assert.AreEqual("1 year", age);

    age = HowOld(new DateTime(2011, 11, 30), new DateTime(2012, 11, 30));
    Assert.AreEqual("1 year", age);

    age = HowOld(new DateTime(2001, 1, 1), new DateTime(2012, 11, 30));
    Assert.AreEqual("11 years", age);

    age = HowOld(new DateTime(2012, 1, 1), new DateTime(2012, 11, 30));
    Assert.AreEqual("10 months", age);

    age = HowOld(new DateTime(2011, 12, 1), new DateTime(2012, 11, 30));
    Assert.AreEqual("11 months", age);

    age = HowOld(new DateTime(2012, 10, 1), new DateTime(2012, 11, 30));
    Assert.AreEqual("1 month", age);

    age = HowOld(new DateTime(2008, 2, 28), new DateTime(2009, 2, 28));
    Assert.AreEqual("1 year", age);

    age = HowOld(new DateTime(2008, 3, 28), new DateTime(2009, 2, 28));
    Assert.AreEqual("11 months", age);

    age = HowOld(new DateTime(2008, 3, 28), new DateTime(2009, 3, 28));
    Assert.AreEqual("1 year", age);

    age = HowOld(new DateTime(2009, 1, 28), new DateTime(2009, 2, 28));
    Assert.AreEqual("1 month", age);

    age = HowOld(new DateTime(2009, 2, 1), new DateTime(2009, 3, 1));
    Assert.AreEqual("1 month", age);

    // NOTE.
    // new DateTime(2008, 1, 31).AddMonths(1) == new DateTime(2009, 2, 28);
    // new DateTime(2008, 1, 28).AddMonths(1) == new DateTime(2009, 2, 28);
    age = HowOld(new DateTime(2009, 1, 31), new DateTime(2009, 2, 28));
    Assert.AreEqual("4 weeks", age);

    age = HowOld(new DateTime(2009, 2, 1), new DateTime(2009, 2, 28));
    Assert.AreEqual("3 weeks", age);

    age = HowOld(new DateTime(2009, 2, 1), new DateTime(2009, 3, 1));
    Assert.AreEqual("1 month", age);

    age = HowOld(new DateTime(2012, 11, 5), new DateTime(2012, 11, 30));
    Assert.AreEqual("3 weeks", age);

    age = HowOld(new DateTime(2012, 11, 1), new DateTime(2012, 11, 30));
    Assert.AreEqual("4 weeks", age);

    age = HowOld(new DateTime(2012, 11, 20), new DateTime(2012, 11, 30));
    Assert.AreEqual("1 week", age);

    age = HowOld(new DateTime(2012, 11, 25), new DateTime(2012, 11, 30));
    Assert.AreEqual("5 days", age);

    age = HowOld(new DateTime(2012, 11, 29), new DateTime(2012, 11, 30));
    Assert.AreEqual("1 day", age);

    age = HowOld(new DateTime(2012, 11, 30), new DateTime(2012, 11, 30));
    Assert.AreEqual("just born", age);

    age = HowOld(new DateTime(2000, 2, 29), new DateTime(2009, 2, 28));
    Assert.AreEqual("8 years", age);

    age = HowOld(new DateTime(2000, 2, 29), new DateTime(2009, 3, 1));
    Assert.AreEqual("9 years", age);

    Exception e = null;

    try
    {
        age = HowOld(new DateTime(2012, 12, 1), new DateTime(2012, 11, 30));
    }
    catch (ArgumentOutOfRangeException ex)
    {
        e = ex;
    }

    Assert.IsTrue(e != null);
}

希望这有帮助。

简单易读，方法互补

public static int getAge(DateTime birthDate)
{
    var today = DateTime.Today;
    var age = today.Year - birthDate.Year;
    var monthDiff = today.Month - birthDate.Month;
    var dayDiff = today.Day - birthDate.Day;

    if (dayDiff < 0)
    {
        monthDiff--;
    }
    if (monthDiff < 0)
    {
       age--;
    }
    return age;
}

这里有一个DateTime扩展程序，它将年龄计算添加到DateTime对象。

public static class AgeExtender
{
    public static int GetAge(this DateTime dt)
    {
        int d = int.Parse(dt.ToString("yyyyMMdd"));
        int t = int.Parse(DateTime.Today.ToString("yyyyMMdd"));
        return (t-d)/10000;
    }
}

对此的简单答案是应用AddYears，如下所示，因为这是唯一一种将年份添加到闰年2月29日的本地方法，并获得普通年份2月28日的正确结果。

有些人认为3月1日是勒普林斯的生日，但.Net和任何官方规则都不支持这一点，也没有常见的逻辑解释为什么一些出生在2月的人应该在另一个月拥有75%的生日。

此外，Age方法可以作为DateTime的扩展添加。由此，您可以以最简单的方式获得年龄：

列表项目

int age=出生日期.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;
    }
}

现在，运行此测试：

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();
    }
}

关键日期示例如下：

出生日期：2000-02-29出生日期：2011-02-28年龄：11

输出：

{
    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
}

2012年2月28日晚些时候：

{
    Birth date: 2000-02-28  Later date: 2012-02-28  Age: 12
    Birth date: 2000-02-28  Later date: 2012-02-29  Age: 12
    Birth date: 2000-02-28  Later date: 2012-03-01  Age: 12
    Birth date: 2000-02-29  Later date: 2012-02-28  Age: 11
    Birth date: 2000-02-29  Later date: 2012-02-29  Age: 12
    Birth date: 2000-02-29  Later date: 2012-03-01  Age: 12
    Birth date: 2000-03-01  Later date: 2012-02-28  Age: 11
    Birth date: 2000-03-01  Later date: 2012-02-29  Age: 11
    Birth date: 2000-03-01  Later date: 2012-03-01  Age: 12
}

下面是一个测试片段：

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
                CalculateAgeCorrect2(bDay, now)));  // outputs 8

这里有一些方法：

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;
}

public int CalculateAgeCorrect2(DateTime birthDate, DateTime now)
{
    int age = now.Year - birthDate.Year;

    // For leap years we need this
    if (birthDate > now.AddYears(-age)) 
        age--;
    // Don't use:
    // if (birthDate.AddYears(age) > now) 
    //     age--;

    return age;
}