How do I calculate someone's age in C#?

Question

Given a DateTime representing their birthday, how do I calculate someone's age?

Answer 1

Many years ago, to provide an age calculator gimmick on my website, I wrote a function to calculate age to a fraction. This is a quick port of that function to C# (from the PHP version). I'm afraid I haven't been able to test the C# version, but hope you enjoy all the same!

(Admittedly this is a bit gimmicky for the purposes of showing user profiles on Stack Overflow, but maybe readers will find some use for it. :-))

double AgeDiff(DateTime date1, DateTime date2) {
    double years = date2.Year - date1.Year;

    /*
     * If date2 and date1 + round(date2 - date1) are on different sides
     * of 29 February, then our partial year is considered to have 366
     * days total, otherwise it's 365. Note that 59 is the day number
     * of 29 Feb.
     */
    double fraction = 365
            + (DateTime.IsLeapYear(date2.Year) && date2.DayOfYear >= 59
            && (date1.DayOfYear < 59 || date1.DayOfYear > date2.DayOfYear)
            ? 1 : 0);

    /*
     * The only really nontrivial case is if date1 is in a leap year,
     * and date2 is not. So let's handle the others first.
     */
    if (DateTime.IsLeapYear(date2.Year) == DateTime.IsLeapYear(date1.Year))
        return years + (date2.DayOfYear - date1.DayOfYear) / fraction;

    /*
     * If date2 is in a leap year, but date1 is not and is March or
     * beyond, shift up by a day.
     */
    if (DateTime.IsLeapYear(date2.Year)) {
        return years + (date2.DayOfYear - date1.DayOfYear
                - (date1.DayOfYear >= 59 ? 1 : 0)) / fraction;
    }

    /*
     * If date1 is not on 29 February, shift down date1 by a day if
     * March or later. Proceed normally.
     */
    if (date1.DayOfYear != 59) {
        return years + (date2.DayOfYear - date1.DayOfYear
                + (date1.DayOfYear > 59 ? 1 : 0)) / fraction;
    }

    /*
     * Okay, here date1 is on 29 February, and date2 is not on a leap
     * year. What to do now? On 28 Feb in date2's year, the ``age''
     * should be just shy of a whole number, and on 1 Mar should be
     * just over. Perhaps the easiest way is to a point halfway
     * between those two: 58.5.
     */
    return years + (date2.DayOfYear - 58.5) / fraction;
}

Answer 2

The best way that I know of because of leap years and everything is:

DateTime birthDate = new DateTime(2000,3,1);
int age = (int)Math.Floor((DateTime.Now - birthDate).TotalDays / 365.25D);

Hope this helps.

Answer 3

@Nick: That's not a correct answer, because like you say, there are leap years, and therefore not each year has 365 days. By just counting the number of days and dividing by 365, you'll get slippage every 4 years or so.

Answer 4

@Nick: No, remember, it matters that one's age updates on one's birthday, and not 1 day early every 4 years or so. While your description of the general age would indeed be correct, the boundary cases here involve days near people's birthdays.

Answer 5

Let's think about the edge cases. In which time zone was the someone born? In which time zone is he or she in today? What if s/he crosses a time zone border within an hour of local time / server time / Greenwich Mean time??!

Answer 6

Another function, not my me but found on the web and a bit refined:

public static int GetAge(DateTime birthDate)
{
    DateTime n = DateTime.Now; // To avoid a race condition around midnight
    int age = DateTime.Now.Year - birthDate.Year;

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

    return age;
}

Just two things that come into my mind: What about people from countries that do not use the gregorian calendar? DateTime.Now is in the server-specific culture i think. I have absolutely 0 knowledge about actually working with Asian calendars and I do not know if there is an easy way to convert dates between calendars, but just in case you're wondering about those chinese guys from the year 4660 :-)

Answer 7

For some reason Jeff's code didn't seem simple enough. To me this seems simpler and easier to understand:

DateTime now = DateTime.Today;
int age = now.Year - bday.Year;
if (bday > now.AddYears(-age)) age--;

Answer 8

Since code wants to be wrong, a solution that solves this problem in more than a few lines is a problem. I think Jeff's original solution or Mike Polen's slightly revised solution are best.

The others are over-engineered.

Answer 9

Mike's is the best answer so far and may actually be pretty close to optimal. Everything else is over-engineered yet still not sufficiently accurate. Some of these solutions miss situations like Feb. 29th birthdays, years with leap seconds, etc. Mike's takes all of those into account through the DateTime class and is very concise and readable as well.

Answer 10

This is the version we use here. It works, and it's fairly simple. It's the same idea as Jeff's but I think it's a little clearer because it separates out the logic for subtracting one, so it's a little easier to understand.

public static int GetAge(this DateTime dateOfBirth, DateTime dateAsAt)
{
    return dateAsAt.Year - dateOfBirth.Year - (dateOfBirth.DayOfYear < dateAsAt.DayOfYear ? 0 : 1);
}

You could expand the ternary operator to make it even clearer, if you think that sort of thing is unclear.

Obviously this is done as an extension method on DateTime, but clearly you can grab that one line of code that does the work and put it anywhere. Here we have another overload of the Extension method that passes in DateTime.Now, just for completeness.

Answer 11

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.

Answer 12

I don't think any of the answers so far provide for cultures that calculate age differently. See, for example, East Asian Age Reckoning versus that in the West.

Any real answer has to include localization. The Strategy Pattern would probably be in order in this example.

Answer 13

I have created a SQL Server User Defined Function to calculate someone's age, given their birthdate. This is useful when you need it as part of a query:

using System;
using System.Data;
using System.Data.Sql;
using System.Data.SqlClient;
using System.Data.SqlTypes;
using Microsoft.SqlServer.Server;

public partial class UserDefinedFunctions
{
    [SqlFunction(DataAccess = DataAccessKind.Read)]
    public static SqlInt32 CalculateAge(string strBirthDate)
    {
        DateTime dtBirthDate = new DateTime();
        dtBirthDate = Convert.ToDateTime(strBirthDate);
        DateTime dtToday = DateTime.Now;

        // get the difference in years
        int years = dtToday.Year - dtBirthDate.Year;
        // subtract another year if we're before the
        // birth day in the current year
        if (dtToday.Month < dtBirthDate.Month || (dtToday.Month == dtBirthDate.Month && dtToday.Day < dtBirthDate.Day))
            years--;
        int intCustomerAge = years;
        return intCustomerAge;
    }
};

Answer 14

I think the TimeSpan has all that we need in it, without having to resort to 365.25 (or any other approximation). Expanding on Aug's example:

DateTime myBD = new DateTime(1980, 10, 10);

TimeSpan difference = DateTime.Now.Subtract(myBD);

textBox1.Text = difference.Years + " years " + difference.Months + " Months " + difference.Days + " days";

Answer 15

My suggestion

int age = (int) ((DateTime.Now - bday).TotalDays/365.255);

That seems to have the year changing on the right date. (I spot tested up to age 107)

Answer 16

When talking about ages, I make certain that I test with leap years. Calculating age, that includes days, isn't so easy I think.

If your birthday is 02/29/1964 how old will you be on 3/1/2009 in years and days?

This is a good test I think.

Answer 17

I've spent some time working on this and came up with this to calculate someone's age in years, months and days. I've tested against the Feb 29th problem and leap years and it seems to work, I'd appreciate any feedback:

public void LoopAge(DateTime myDOB, DateTime FutureDate)
{
    int years = 0;
    int months = 0;
    int days = 0;

    DateTime tmpMyDOB = new DateTime(myDOB.Year, myDOB.Month, 1);

    DateTime tmpFutureDate = new DateTime(FutureDate.Year, FutureDate.Month, 1);

    while (tmpMyDOB.AddYears(years).AddMonths(months) < tmpFutureDate)
    {
        months++;
        if (months > 12)
        {
            years++;
            months = months - 12;
        }
    }

    if (FutureDate.Day >= myDOB.Day)
    {
        days = days + FutureDate.Day - myDOB.Day;
    }
    else
    {
        months--;
        if (months < 0)
        {
            years--;
            months = months + 12;
        }
        days +=
            DateTime.DaysInMonth(
                FutureDate.AddMonths(-1).Year, FutureDate.AddMonths(-1).Month
            ) + FutureDate.Day - myDOB.Day;

    }

    //add an extra day if the dob is a leap day
    if (DateTime.IsLeapYear(myDOB.Year) && myDOB.Month == 2 && myDOB.Day == 29)
    {
        //but only if the future date is less than 1st March
        if (FutureDate >= new DateTime(FutureDate.Year, 3, 1))
            days++;
    }

}

Answer 18

I am late to the party, but here's a one-liner:

int age = new DateTime(DateTime.Now.Subtract(birthday).Ticks).Year-1;

Answer 19

Here is a solution.

    DateTime dateOfBirth = new DateTime(2000, 4, 18);
    DateTime currentDate = DateTime.Now;

    int ageInYears = 0;
    int ageInMonths = 0;
    int ageInDays = 0;

    ageInDays = currentDate.Day - dateOfBirth.Day;
    ageInMonths = currentDate.Month - dateOfBirth.Month;
    ageInYears = currentDate.Year - dateOfBirth.Year;

    if (ageInDays < 0)
    {
        ageInDays += DateTime.DaysInMonth(currentDate.Year, currentDate.Month);
        ageInMonths = ageInMonths--;

        if (ageInMonths < 0)
        {
            ageInMonths += 12;
            ageInYears--;
        }
    }
    if (ageInMonths < 0)
    {
        ageInMonths += 12;
        ageInYears--;
    }

    Console.WriteLine("{0}, {1}, {2}", ageInYears, ageInMonths, ageInDays);

Answer 20

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),
   CalculateAgeWrong2(bDay, now),
   CalculateAgeCorrect(bDay, now)));

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;
}

Answer 21

Would this work?

public override bool IsValid(DateTime value)
        {
            _dateOfBirth =  value;

            var yearsOld = (double) (DateTime.Now.Subtract(_dateOfBirth).TotalDays/365);

            if (yearsOld > 18)
                return true;
            return false; 
        }

Answer 22

I've created an Age struct, which looks like this:

public struct Age : IEquatable<Age>, IComparable<Age>
{
    private readonly int _years;
    private readonly int _months;
    private readonly int _days;

    public int Years  { get { return _years; } }
    public int Months { get { return _months; } }
    public int Days { get { return _days; } }

    public Age( int years, int months, int days ) : this()
    {
        _years = years;
        _months = months;
        _days = days;
    }

    public static Age CalculateAge( DateTime dateOfBirth, DateTime date )
    {
        // Here is some logic that ressembles Mike's solution, although it
        // also takes into account months & days.
        // Ommitted for brevity.
        return new Age (years, months, days);
    }

    // Ommited Equality, Comparable, GetHashCode, functionality for brevity.
}

Answer 23

Here's a little code sample for C# I knocked up, be careful around the edge cases specifically leap years, not all the above solutions take them into account. Pushing the answer out as a DateTime can cause problems as you could end up trying to put too many days into a specific month e.g. 30 days in Feb.

public string LoopAge(DateTime myDOB, DateTime FutureDate)
{
    int years = 0;
    int months = 0;
    int days = 0;

    DateTime tmpMyDOB = new DateTime(myDOB.Year, myDOB.Month, 1);

    DateTime tmpFutureDate = new DateTime(FutureDate.Year, FutureDate.Month, 1);

    while (tmpMyDOB.AddYears(years).AddMonths(months) < tmpFutureDate)
    {
     months++;
     if (months > 12)
     {
      years++;
      months = months - 12;
     }
    }

    if (FutureDate.Day >= myDOB.Day)
    {
     days = days + FutureDate.Day - myDOB.Day;
    }
    else
    {
     months--;
     if (months < 0)
     {
      years--;
      months = months + 12;
     }
     days = days + (DateTime.DaysInMonth(FutureDate.AddMonths(-1).Year, FutureDate.AddMonths(-1).Month) + FutureDate.Day) - myDOB.Day;

    }

    //add an extra day if the dob is a leap day
    if (DateTime.IsLeapYear(myDOB.Year) && myDOB.Month == 2 && myDOB.Day == 29)
    {
     //but only if the future date is less than 1st March
     if(FutureDate >= new DateTime(FutureDate.Year, 3,1))
      days++;
    }

    return "Years: " + years + " Months: " + months + " Days: " + days;
}

Answer 24

I use this:

public static class DateTimeExtensions
{
    public static int Age(this DateTime birthDate)
    {
        return Age(birthDate, DateTime.Now);
    }
    public static int Age(this DateTime birthDate, DateTime offsetDate)
    {
        int result;

        result = offsetDate.Year - birthDate.Year;
        if (offsetDate.DayOfYear < birthDate.DayOfYear) result--;

        return result;
    }
}

Answer 25

Keeping it simple (and possibly stupid:)).

DateTime birth = new DateTime(1975, 09, 27, 01, 00, 00, 00);
TimeSpan ts = DateTime.Now - birth;
Console.WriteLine("You are approximately " + ts.TotalSeconds.ToString() + " seconds old.");

Answer 26

   private int GetAge(int _year, int _month, int _day
    {
            DateTime yourBirthDate= new DateTime(_year, _month, _day);

            DateTime todaysDateTime = DateTime.Today;
            int noOfYears = todaysDateTime.Year - yourBirthDate.Year;
            if (DateTime.Now.Month < yourBirthDate.Month ||
 (DateTime.Now.Month == yourBirthDate.Month && DateTime.Now.Day < yourBirthDate.Day))
            {
                noOfYears--;
            }
            return  noOfYears;
    }

Answer 27

The simplest way I've ever found is this. It works correctly for the US and western europe locales. Can't speak to other locales, especially places like China. 4 extra compares, at most, following the initial computation of age.

public int AgeInYears( DateTime birthDate , DateTime referenceDate )
{
  Debug.Assert( 
      referenceDate >= birthDate , 
      "birth date must be on or prior to the reference date" ) ;

  DateTime birth     = birthDate.Date     ;
  DateTime reference = referenceDate.Date ;
  int      years     = ( reference.Year - birth.Year ) ;

  //
  // an offset of -1 is applied if the birth date has 
  // not yet occurred in the current year.
  //
  if      ( reference.Month > birth.Month )         ;
  else if ( reference.Month < birth.Month ) --years ;
  else // in birth month
  {
    if ( reference.Day < birth.Day ) --years ;
  }

  return years ;

}