# Calculate Age with a single line of code C#

• ### Question

• ```int age = Convert.ToInt32 (Math.Round (DateTime.Now.Subtract (dateOfBirth).TotalDays * 0.00273790926 ));
```

• Edited by Wednesday, October 26, 2011 4:12 AM
• Changed type Wednesday, October 26, 2011 3:18 PM : Post is a Question :
Wednesday, October 26, 2011 4:08 AM

• I am pretty sure the following can be refactored into one line of code, if you so desire.

```        public int ValidateBirthDate(DateTime birthday)
{
DateTime today = DateTime.Now;
DateTime validDate = new DateTime(today.Year - 21, today.Month, today.Day);
TimeSpan validAge = today.Subtract(validDate);
TimeSpan actualAge = today.Subtract(birthday);
return TimeSpan.Compare(validAge, actualAge);
}
```

Rudy   =8^D

Mark the best replies as answers. "Fooling computers since 1971."

http://thesharpercoder.blogspot.com/

Wednesday, October 26, 2011 3:20 PM
• Hi Louis,

Yeah, I forget about the Leap Year.....The Below approach solves it i hope

DateTime birthday = new DateTime(1991,03,12);

int age = ((DateTime.Now.Year - birthday.Year) * 372 + (DateTime.Now.Month - birthday.Month) * 31 + (DateTime.Now.Day - birthday.Day)) / 372;

return age.ToString();

Regards, Rajasekhar.R
Thursday, October 27, 2011 11:27 AM
• A few tests return correct results, but I would rather use simpler-to-read tests instead of multiplications and divisions:
```int age = DateTime.Now.Year - birthday.Year;
if (DateTime.Now.Month < birthday.Month || DateTime.Now.Month == birthday.Month && DateTime.Now.Day < birthday.Day) age--;
```

• Proposed as answer by Thursday, October 27, 2011 4:58 PM
• Marked as answer by Thursday, November 3, 2011 5:20 AM
Thursday, October 27, 2011 3:14 PM
• Curiosity got the better of me.

```
private void button2_Click(object sender, EventArgs e)
{
DateTime birthday = DateTime.Parse(this.textBox4.Text);  // write birthday as "11/26/1990"
this.textBox5.Text = this.calculator.ValidateBirthDate(birthday).ToString();
this.textBox6.Text = this.calculator.CheckBirthday(birthday).ToString();
}

/// <summary>
/// Checks to see if birthday is at least 21 years of age
/// </summary>
/// <param name="birthday">Enter birthday as "mm/dd/yyyy"</param>
/// <returns>Returns 0 or -1 if birthday is least 21.  Returns 1 if under 21. </returns>
public int CheckBirthday(DateTime birthday)
{
return TimeSpan.Compare((DateTime.Now.Subtract(new DateTime(DateTime.Now.Year - 21, DateTime.Now.Month, DateTime.Now.Day))), DateTime.Now.Subtract(birthday));
}```

I would rather let the Compiler refactor it into one line for me.

Rudy   =8^D

Mark the best replies as answers. "Fooling computers since 1971."

http://thesharpercoder.blogspot.com/

Wednesday, October 26, 2011 3:56 PM

### All replies

• Hi CodeSnorter,

I think using Math.Round method could not give you the exact age value because from DateTime.Now.Subtract(dateOfBirth).TotalDays * the number of days in a year, produces a decimal value then rounded off to a whole number. For example my birthdate is jan 29, 1983, the actual output if we convert it to decimal is 28.74 then rounded off to whole number in which 29 is the result, which is not my exact age today. So the better method for this is to use Math.Truncate:

```int age = Convert.ToInt32(Math.Truncate(DateTime.Now.Subtract(dateOfBirth).TotalDays * (1 / 365.242199)));
```

Wednesday, October 26, 2011 7:30 AM
• Hi CodeSnorter,

I fear that this is not 100% correct because you are not handling the years correctly. So it might lead to slightly wrong results.

Exampel:

DateTime birthday = new DateTime(1970, 1, 1);
DateTime now1 = new DateTime(2011, 12, 31, 23, 59, 59);
DateTime now2 = new DateTime(2012, 1, 1, 0, 0, 1);

int age1 = Convert.ToInt32(Math.Round(now1.Subtract(birthday).TotalDays * 0.00273790926));
int age2 = Convert.ToInt32(Math.Round(now2.Subtract(birthday).TotalDays * 0.00273790926));

To times 42, but as you can see: at now1 he was still 41 years old because 1 second was left till his birthday but the code already tells me: his age is 42.

So maybe it could be better to use a function like DateAndTime.DateDiff (ok, this comes from the Microsoft.VisualBasic Namespace which c# developers often like to avoid.) which I didn't test in detail.

In general, I would create a small function instead that calculates the difference in years (Using DateTime.Year Property) and then evaluates the Month/Day (probably the hour/minutes/seconds if you want to be that exact but normaly you do not care when you was born exactly. Just the day is important!) to see if you have to subtract 1. (e.g. birthday is 03.03.1972 and it is the 02.02.1975)  : 1975 - 1972 = 3. But because it is before the 03.03 you have to remove 1.

And of course: You cannot use the TimeSpan. The TimeSpan can only work with days. It cannot work with months or years, because a timespan of 1 Year or 1 Month is not fixed. 1 Month can be 27, 28, 30 or 31 days depending on the month and year. And a year is also not a fixed size. (And that is also the reason, why your calculation cannot be exact!)

With kind regards,

Wednesday, October 26, 2011 8:00 AM
• Hi,

this does not really fix the main problem. If you use Truncate, my example will show an age of 41 years instead of 42 years in both cases. So calculating the age that way could result in some mad people that try to get their driving license after reaching the required age and the computer says: Sorry - you are not old enough :)

So I would not calculate the age that way.

With kind regards,

Wednesday, October 26, 2011 8:03 AM
• It should be 1 / 365.2425, or better subtract the years and subtract 1 if the date has not been reached.
Wednesday, October 26, 2011 8:06 AM
• Hi,

this does not really fix the main problem. If you use Truncate, my example will show an age of 41 years instead of 42 years in both cases. So calculating the age that way could result in some mad people that try to get their driving license after reaching the required age and the computer says: Sorry - you are not old enough :)

So I would not calculate the age that way.

With kind regards,

Yes you're right, I done a lot of testing using truncate with datetimepicker as input but there's still a problem with it. So I have tried to convert the dateOfBirth value to dateOfBirth = DateTime.Parse(dateTimePicker1.Value.ToLongDateString()), which provides me the exact age calculation but I think it is only concern with date not with time.:)

Thanks

Wednesday, October 26, 2011 8:32 AM
• I fear that you cannot calculate it that way. I will try to explain it to you a little.

You get the difference in days between your 2 dates. But you are unable to exactly tell the number of years or months from the number of days, because it is not fixed.

So if you look at months:

Starting from 01.01.2011 you need 31 days to get to the next month. So 1 month is 31 days.
Starting from 01.04.2011 you need 30 days only to get tho the next month. So 1 month is 30 days.

So how many days is a month now? You can calculate with an average. So it will be like 30,xxxxxx days. But as soon as you calculate with that, you will get wrong results: Starting from 01.01.2011 you will go these days and you will end before the next month. And starting from 01.04.2011 you will end after the month finished already. (And when you take the february it start to be much worse!)

Same with years, because the number of days per year is not fixed.

So you will always end up with something that is more or less correct.

The code to calculate the exact age is not complex. So it is not worth losing the correct information because of some "easier code". (But the code is much harder to read. And we already have some discussions what the correct average length of a year is ...)

With kind regards,

Wednesday, October 26, 2011 8:59 AM
• I am pretty sure the following can be refactored into one line of code, if you so desire.

```        public int ValidateBirthDate(DateTime birthday)
{
DateTime today = DateTime.Now;
DateTime validDate = new DateTime(today.Year - 21, today.Month, today.Day);
TimeSpan validAge = today.Subtract(validDate);
TimeSpan actualAge = today.Subtract(birthday);
return TimeSpan.Compare(validAge, actualAge);
}
```

Rudy   =8^D

Mark the best replies as answers. "Fooling computers since 1971."

http://thesharpercoder.blogspot.com/

Wednesday, October 26, 2011 3:20 PM
• Curiosity got the better of me.

```
private void button2_Click(object sender, EventArgs e)
{
DateTime birthday = DateTime.Parse(this.textBox4.Text);  // write birthday as "11/26/1990"
this.textBox5.Text = this.calculator.ValidateBirthDate(birthday).ToString();
this.textBox6.Text = this.calculator.CheckBirthday(birthday).ToString();
}

/// <summary>
/// Checks to see if birthday is at least 21 years of age
/// </summary>
/// <param name="birthday">Enter birthday as "mm/dd/yyyy"</param>
/// <returns>Returns 0 or -1 if birthday is least 21.  Returns 1 if under 21. </returns>
public int CheckBirthday(DateTime birthday)
{
return TimeSpan.Compare((DateTime.Now.Subtract(new DateTime(DateTime.Now.Year - 21, DateTime.Now.Month, DateTime.Now.Day))), DateTime.Now.Subtract(birthday));
}```

I would rather let the Compiler refactor it into one line for me.

Rudy   =8^D

Mark the best replies as answers. "Fooling computers since 1971."

http://thesharpercoder.blogspot.com/

Wednesday, October 26, 2011 3:56 PM
• Hi,

Try This  Simple one...

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

Regards,

Rajasekhar.R

Thursday, October 27, 2011 7:38 AM
•  int age= new DateTime(DateTime.Now.Subtract(birthday).Ticks).year-1;

Did you thought about what happens for leap years?
Thursday, October 27, 2011 10:23 AM
• Hi Louis,

Yeah, I forget about the Leap Year.....The Below approach solves it i hope

DateTime birthday = new DateTime(1991,03,12);

int age = ((DateTime.Now.Year - birthday.Year) * 372 + (DateTime.Now.Month - birthday.Month) * 31 + (DateTime.Now.Day - birthday.Day)) / 372;

return age.ToString();

Regards, Rajasekhar.R
Thursday, October 27, 2011 11:27 AM
• A few tests return correct results, but I would rather use simpler-to-read tests instead of multiplications and divisions:
```int age = DateTime.Now.Year - birthday.Year;
if (DateTime.Now.Month < birthday.Month || DateTime.Now.Month == birthday.Month && DateTime.Now.Day < birthday.Day) age--;
```

• Proposed as answer by Thursday, October 27, 2011 4:58 PM
• Marked as answer by Thursday, November 3, 2011 5:20 AM
Thursday, October 27, 2011 3:14 PM
• Here's the more important question that hasn't been asked yet (as far as I can tell)...

Why the heck do you want to do this in one line? Never try to make a problem shorter (or longer, but that's a different discussion) than its complexity demands. Calculating age is not a one-line kind of question.

Take several lines to calculate age, so it is readable and maintainable (like rudedog's first post). Put that in a method, and call it wherever you need it. The method call is still 1 line, but you don't have to feel like beating your head against the wall any time you have to read or maintain it.

Check out My Blog for tech news, development tips, and other information for geeks like me.
• Proposed as answer by Friday, October 28, 2011 3:37 AM
Thursday, October 27, 2011 3:24 PM
• I must be missing something obvious.  Why is everyone so worried about leap years?  Why are all of you trying to re-invent the wheel?  You don't need to write code to compensate for leap years.  The DateTime type contains member methods that already compensate for leap years, leap days, leap hours, etc.  Test it and see for yourselves.

public int GetAgeInDays(DateTime birthday)
{
return DateTime.Now.Subtract(birthday).Days;
}

Try calling that method with today's date minus 1 year, and you get 365,  Then try it again with minus 2 years (730 days), with minus 3 years (1095 days), and finally with minus 4 years you get 1461 days.

Mark the best replies as answers. "Fooling computers since 1971."

http://thesharpercoder.blogspot.com/

Thursday, October 27, 2011 9:56 PM
• Hi Rudy,

the problem why the leap year is important is simply, that the TE was using this number of days to calculate back to the year.

And that is simply not possible because it depends on the leap years. That is, what I tried to tell first and what louise.fr explained very nicely.

And that is also the reason, why TimeSpan is not giving any higher values than days:
"The largest unit of time that the TimeSpan structure uses to measure duration is a day. Time intervals are measured in days for consistency, because the number of days in larger units of time, such as months and years, varies."

So the valid and easy to understand code to calculate the year is simply a compare between the year and then check if 1 must be substracted by checking the months / months + days and not to try to calculate years from a timespan.

With kind regards,

Friday, October 28, 2011 3:35 AM
• Here's a sample program that shows that Louis's and Rajasekhar.R's algorithms both produce the same results, but just using TimeSpan.TotalDays doesn't agree:

```using System;

namespace Demo
{
class Program
{
static void Main(string[] args)
{
DateTime baseBirthday = new DateTime(1961, 07, 19, 12, 0, 0);
DateTime baseToday    = new DateTime(1970, 08, 16, 12, 0, 0);

for (int birthdayOffset = 0; birthdayOffset < 365*5; ++birthdayOffset)
{
DateTime birthday = baseBirthday + TimeSpan.FromDays(birthdayOffset);

for (int todayOffset = 0; todayOffset < 365*5; ++todayOffset)
{
DateTime today = baseToday + TimeSpan.FromDays(todayOffset);
int age1 = Age1(birthday, today);
int age2 = Age2(birthday, today);
int age3 = Age3(birthday, today);

if (age1 != age2 /*|| age1 != age3*/)  // Uncomment the "|| age1 != age3" to make it fail.
{
Console.WriteLine("Failure with birthday = " + birthday + " and today = " + today);
Console.WriteLine("age1 = " + age1 + ", age2 = " + age2 + ", age3 = " + age3);
return;
}
}
}

Console.WriteLine("Success");
}

static int Age1(DateTime birthday, DateTime today)
{
int age = today.Year - birthday.Year;

if (today.Month < birthday.Month || today.Month == birthday.Month && today.Day < birthday.Day)
{
--age;
}

return age;
}

static int Age2(DateTime birthday, DateTime today)
{
return ((today.Year - birthday.Year) * 372 + (today.Month - birthday.Month) * 31 + (today.Day - birthday.Day)) / 372;
}

static int Age3(DateTime birthday, DateTime now)
{
// THIS IS WRONG:
double ageInDays = now.Subtract(birthday).TotalDays;
return (int)(ageInDays/365.25); // Also wrong if you divide by 365.0
}
}
}

```

Friday, October 28, 2011 8:56 AM
•

EDITMatthew,

I ran your code several times, and it never hit the line of code to report a failure.  What are we trying to calculate here?  A person's age from total days of age?  Total days from a person's age or birthday?   I still don't understand what is the issue under concern here.

The code that I posted performs the math using the type DateTime, not TimeSpan.  (Never knew there was an apparent issue with TimeSpan, which seems highly irregular to me.)  So far, it seems to always come up with the right number of days or years.

Rudy   =8^D

Mark the best replies as answers. "Fooling computers since 1971."

http://thesharpercoder.blogspot.com/

EDIT:  I ran the code snippet on an old Pentium 4 running XP Pro SP3, running VS2010 C# Express.

Friday, October 28, 2011 1:56 PM
• Did you uncomment the part of the line mentioned in this line?

`if (age1 != age2 /*|| age1 != age3*/)  // Uncomment the "|| age1 != age3" to make it fail.`

You need to make it look like this:

```if (age1 != age2 || age1 != age3)
```

The comparison you make is slightly different, but I think it won't work for some cases. I'll try to modify the code to test it, but in the meantime can you write a method that returns the age of someone given a birth date and today's date (passing in today's date to the method rather than using DateTime.Now inside the function)?

>>The code that I posted performs the math using the type DateTime, not TimeSpan

What about the "DateTime.Now.Subtract(birthday)"? That returns a TimeSpan, which you are using.

Anyway, what we're trying to calculate is how old someone is in years given a birthdate and today's date, that's all.

Your age in years increments on each of your birthdays, of course! :)

[EDIT] I was unable to test your code Rudy, because check this bit:

`new DateTime(DateTime.Now.Year - 21, DateTime.Now.Month, DateTime.Now.Day)`

Imagine that today is February 29th on a leap year, for example 29th Feb 1972.

You will try to create a DateTime with year = 1972-21 = 1951, Month = February, Day = 29. This blows up because 1951 wasn't a leap year. (Obviously there are many other cases where this happens; 1972 was just an example).

Friday, October 28, 2011 2:46 PM
• The code that I posted performs the math using the type DateTime, not TimeSpan.  (Never knew there was an apparent issue with TimeSpan, which seems highly irregular to me.)  So far, it seems to always come up with the right number of days or years.

You compute the number of days, and that sure returns the correct number, but I don't see where you compute the number of years.

TimeSpan doesn't have an issue. It is a time span, independent of a starting point, and therefore doesn't have a Months or Years property.

Friday, October 28, 2011 2:48 PM
• Is there really anything wrong with the 3-line solution posted here on SO?

Calculating age is a problem that has been solved a billion times over in every possible programming language. No need to reinvent the wheel - google search will usually give you a pretty simple answer. If you just need to calculate an age, you can find that in about 10 seconds (ignoring the one-line requirement, which, frankly, is a stupid requirement). Seems like this has garnered way more discussion than the problem warrants.

Check out My Blog for tech news, development tips, and other information for geeks like me.
Friday, October 28, 2011 3:06 PM
• Just because other people have discussed something in detail, it doesn't mean nobody else is allowed to. :)

I think Rajasekhar.R's solution is quite interesting:

int age = ((DateTime.Now.Year - birthday.Year) * 372 + (DateTime.Now.Month - birthday.Month) * 31 + (DateTime.Now.Day - birthday.Day)) / 372;

Now, why does that work? :)

Friday, October 28, 2011 3:24 PM
• It certainly doesn't mean people aren't allowed to, but it does mean that it's not really productive. Most of us don't spend time trying to figure out how to implement sine or cosine, because some library already solves it for us. The beauty of using a language like C# is that we don't have to re-solve issues which have already been solved. We can work on larger and more complex issues like business problems.
Check out My Blog for tech news, development tips, and other information for geeks like me.
• Proposed as answer by Friday, October 28, 2011 6:29 PM
• Unproposed as answer by Friday, October 28, 2011 11:33 PM
Friday, October 28, 2011 3:33 PM
• Now, why does that work? :)

Let's set Yn = DateTime.Now.Year, Yb = birthday.Year, Mn = DateTime.Now.Month, Mb = birthday.Month, Dn = DateTime.Now.Day, Db = birthday.Day

age = Yn - Yb + (31*(Mn - Mb) + (Dn - Db)) / 372

We know that what we need is either Yn-Yb if the date has already been reached, Yn-Yb-1 if it has not.

a) If Mn<Mb, we have -341 <= 31*(Mn-Mb) <= -31 and -30 <= Dn-Db <= 30

-371 <= 31*(Mn - Mb) + (Dn - Db) <= -1

With integer division

(31*(Mn - Mb) + (Dn - Db)) / 372 = -1

b) If Mn=Mb and Dn<Db, we have 31*(Mn - Mb) = 0 and -30 <= Dn-Db <= -1

With integer division, again

(31*(Mn - Mb) + (Dn - Db)) / 372 = -1

c) If Mn>Mb, we have 31 <= 31*(Mn-Mb) <= 341 and -30 <= Dn-Db <= 30

1 <= 31*(Mn - Mb) + (Dn - Db) <= 371

With integer division

(31*(Mn - Mb) + (Dn - Db)) / 372 = 0

d) If Mn=Mb and Dn>Db, we have 31*(Mn - Mb) = 0 and 1 <= Dn-Db <= 30

With integer division, again

(31*(Mn - Mb) + (Dn - Db)) / 372 = 0

e) If Mn=Mb and Dn=Db, we have 31*(Mn - Mb) + Dn-Db = 0

and therefore (31*(Mn - Mb) + (Dn - Db)) / 372 = 0

Monday, October 31, 2011 11:17 AM

DateTime birthday;

int age = Convert.ToInt32(Math.Truncate(DateTime.Now.Subtract(birthday=new DateTime(year,month,day)).TotalDays * (1 / 365.242199)));

• Edited by Monday, October 31, 2011 7:09 PM
Monday, October 31, 2011 7:05 PM