How to Calculate Someone'S Age Based on a Datetime Type Birthday

How do I calculate someone's age based on a DateTime type birthday?

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 in which the person was born in case of a leap year
if (birthdate.Date > today.AddYears(-age)) age--;

However, this assumes you are looking for the western idea of the age and not using East Asian reckoning.

How do I calculate someone's age based on a DateTime type birthday?

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 in which the person was born in case of a leap year
if (birthdate.Date > today.AddYears(-age)) age--;

However, this assumes you are looking for the western idea of the age and not using East Asian reckoning.

How to calculate an age based on a birthday

Stack Overflow uses such a function to determine the age of a user.

How do I calculate someone's age based on a DateTime type birthday?

The given answer is

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

So your helper method would look like:

public static string Age(this HtmlHelper helper, DateTime birthday)
{
    DateTime now = DateTime.Today;
    int age = now.Year - birthday.Year;
    if (now < birthday.AddYears(age)) 
        age--;

    return age.ToString();
}

Today, I use a different version of this function to include a date of reference. This allow me to get the age of someone at a future date or in the past. This is used for our reservation system, where the age in the future is needed.

public static int GetAge(DateTime reference, DateTime birthday)
{
    int age = reference.Year - birthday.Year;
    if (reference < birthday.AddYears(age))
        age--;

    return age;
}

How to calculate age (in years) based on Date of Birth and getDate()

There are issues with leap year/days and the following method, see the update below:

try this:

DECLARE @dob  datetime
SET @dob='1992-01-09 00:00:00'

SELECT DATEDIFF(hour,@dob,GETDATE())/8766.0 AS AgeYearsDecimal
    ,CONVERT(int,ROUND(DATEDIFF(hour,@dob,GETDATE())/8766.0,0)) AS AgeYearsIntRound
    ,DATEDIFF(hour,@dob,GETDATE())/8766 AS AgeYearsIntTrunc

OUTPUT:

AgeYearsDecimal                         AgeYearsIntRound AgeYearsIntTrunc
--------------------------------------- ---------------- ----------------
17.767054                               18               17

(1 row(s) affected)

UPDATE here are some more accurate methods:

BEST METHOD FOR YEARS IN INT

DECLARE @Now  datetime, @Dob datetime
SELECT   @Now='1990-05-05', @Dob='1980-05-05'  --results in 10
--SELECT @Now='1990-05-04', @Dob='1980-05-05'  --results in  9
--SELECT @Now='1989-05-06', @Dob='1980-05-05'  --results in  9
--SELECT @Now='1990-05-06', @Dob='1980-05-05'  --results in 10
--SELECT @Now='1990-12-06', @Dob='1980-05-05'  --results in 10
--SELECT @Now='1991-05-04', @Dob='1980-05-05'  --results in 10

SELECT
    (CONVERT(int,CONVERT(char(8),@Now,112))-CONVERT(char(8),@Dob,112))/10000 AS AgeIntYears

you can change the above 10000 to 10000.0 and get decimals, but it will not be as accurate as the method below.

BEST METHOD FOR YEARS IN DECIMAL

DECLARE @Now  datetime, @Dob datetime
SELECT   @Now='1990-05-05', @Dob='1980-05-05' --results in 10.000000000000
--SELECT @Now='1990-05-04', @Dob='1980-05-05' --results in  9.997260273973
--SELECT @Now='1989-05-06', @Dob='1980-05-05' --results in  9.002739726027
--SELECT @Now='1990-05-06', @Dob='1980-05-05' --results in 10.002739726027
--SELECT @Now='1990-12-06', @Dob='1980-05-05' --results in 10.589041095890
--SELECT @Now='1991-05-04', @Dob='1980-05-05' --results in 10.997260273973

SELECT 1.0* DateDiff(yy,@Dob,@Now) 
    +CASE 
         WHEN @Now >= DATEFROMPARTS(DATEPART(yyyy,@Now),DATEPART(m,@Dob),DATEPART(d,@Dob)) THEN  --birthday has happened for the @now year, so add some portion onto the year difference
           (  1.0   --force automatic conversions from int to decimal
              * DATEDIFF(day,DATEFROMPARTS(DATEPART(yyyy,@Now),DATEPART(m,@Dob),DATEPART(d,@Dob)),@Now) --number of days difference between the @Now year birthday and the @Now day
              / DATEDIFF(day,DATEFROMPARTS(DATEPART(yyyy,@Now),1,1),DATEFROMPARTS(DATEPART(yyyy,@Now)+1,1,1)) --number of days in the @Now year
           )
         ELSE  --birthday has not been reached for the last year, so remove some portion of the year difference
           -1 --remove this fractional difference onto the age
           * (  -1.0   --force automatic conversions from int to decimal
                * DATEDIFF(day,DATEFROMPARTS(DATEPART(yyyy,@Now),DATEPART(m,@Dob),DATEPART(d,@Dob)),@Now) --number of days difference between the @Now year birthday and the @Now day
                / DATEDIFF(day,DATEFROMPARTS(DATEPART(yyyy,@Now),1,1),DATEFROMPARTS(DATEPART(yyyy,@Now)+1,1,1)) --number of days in the @Now year
             )
     END AS AgeYearsDecimal

How do I calculate someone's age based on a DateTime type birthday?

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 in which the person was born in case of a leap year
if (birthdate.Date > today.AddYears(-age)) age--;

However, this assumes you are looking for the western idea of the age and not using East Asian reckoning.

c# calculating age using datetime but the given birthdate goes to 01-01-0001

I just tested the code you provided and it works just fine with 1 customer. The problem you have is probably around the list and having the class Customer containing a list of customers and how you access the age of instantiated objects.

This works just fine:

    public class Customer
    {
        private string name;
        private DateTime signUpDate;

        public DateTime BirthDate { get; }

        public int Age
        {
            get
            {
                int Age = (int)(DateTime.Today - BirthDate).TotalDays;
                Age = Age / 365;
                return Age;
            }
        }

        public Customer(string name, DateTime BirthDate, DateTime signUpDate)
        {
            this.name = name;
            this.signUpDate = signUpDate;
            this.BirthDate = BirthDate;
        }
    }

With a button to calculate the age and show:

        private void button1_Click(object sender, EventArgs e)
        {
            Customer test = new Customer("Lionel Messi", new DateTime(2000, 3, 12), new DateTime(2019, 2, 23));

            teAge.Text = test.Age.ToString();
        }

If you want to post exactly how you are accessing the age maybe I can help you more.

Delphi Calculate age of person from TDateEdit component

Here is a function to calculate someone's age.

It is distinctly different from the RTL function YearsBetween, as that calculates the number of years between two dates; and is fundamentally not the same as someone's age.

function GetAge(const BirthDate, CurrentDate: TDateTime): Integer;
var
    y1, m1, d1: Word; //born
    y2, m2, d2: Word; //today
begin
    Result := 0;

    if CurrentDate < BirthDate then
        Exit;

    DecodeDate(BirthDate, y1, m1, d1);
    DecodeDate(CurrentDate, y2, m2, d2);

    //Fudge someone born on the leap-day to Feb 28th of the same year
    //strictly for the purposes of this calculation
    if ( (m1=2) and (d1=29) )
            and
        ( not IsLeapYear(y2) ) then
    begin
        d1 := 28;
    end;

    Result := y2-y1; //rough count of years
    //Take away a year of the month/day is before their birth month/day
    if (m2 < m1) or
            ((m2=m1) and (d2<d1)) then
        Dec(Result);
end;

How to calculate age by input field date on the frontend?

Age should not be stored data - it's a calculated value based on the time of the query; therefore I would recommend making it a read-only property that's calculated on the fly:

public DateTime Birthday { get; set; } = DateTime.Today;

public int Age
{
    get
    {
        var today = DateTime.Today;
        var age = today.Year - Birthday.Year;
        if (Birthday.Date > today.AddYears(-age)) age--;
        return age;
    }
}

Now you have a calculated field that will give the accurate Age (in years), which you can then use to compare against 50.

Note that the age calculation comes from this answer.

Can calculate age with user input in C#, but don't know why it works

You are asking about how this line works:

if (myDate.Date > today.AddYears(-age)) age--;

The number of years has been calculated by subtracting the year from the DOB from the year now. For example, if your DOB was 15/05/1979 and now is 2020 this would give:

age = 2020 - 1979 = 41

However, if this person's DOB was after today, they would not be 41 yet, they would still be 40.

In the case of the if statement, if the date of their birth (the month and day) is after today, one year is subtracted from age to give the correct age (which would be 40 in my example).

The logic is not as concise as it could be, but essentially it says:

• Take the calculated age in years from today's date (example: 15/05/1979 = 41 years, 12/01/2020 - 41 years = 12/01/1979)
• Is the DOB after this value? (is 15/05/1979 AFTER 12/01/1979)
• If yes, subtract 1 from age (example: 41 - 1 = 40 years)

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