Question

My application needs to adjust a clients current age by +0.5 if it has been 6 months since their last birthday.

The code should look something like this, but how many ticks would there be in 6 months?

if (DateTime.Today - dateOfBirth.Date > new TimeSpan(6))
        {
            adjust = 0.5M;
        }
        else
        {
            adjust = 0M;
        }

Thanks in advance

Answer 1

EDIT: You know what, actually? Since clearly what you really need is just to display the user’s age to within 6 months, this is what you should really do.

static decimal GetApproximateAge(DateTime dateOfBirth) {
    TimeSpan age = DateTime.Now - dateOfBirth;

    /* a note on the line below:
     * treating 182.5 days as equivalent to 6 months,
     * reasoning that this will provide acceptable accuracy
     * for ages below ~360 years
     * 
     * (result will be 1 day off for roughly every 4 years;
     * for a calculation of half-years to be inaccurate
     * would take 4 [years] * ~90 [days] = ~360)
     */

    // get age in units of 6 months
    // desired behavior is not to add 0.5 until
    // a full six months have elapsed since the user's last birthday --
    // using Math.Floor to ensure this
    double approxAgeInHalfYears = Math.Floor(age.TotalDays / 182.5);

    // now convert this to years
    // did it this way to restrict age to increments of 0.5
    double approxAgeInYears = approxAgeInHalfYears * 0.5;

    return Convert.ToDecimal(approxAgeInYears);
}

The above code includes a big comment explaining its own shortcomings, helpfully pointed out by David.

Now, here’s yet another option. It’s kind of ugly because it uses a loop; but it’s also more rock-solid since it utilizes the DateTime.AddMonths method, which has the advantage of having already been tested and documented by Microsoft.

static decimal GetApproximateAge(DateTime dateOfBirth) {
    DateTime now = DateTime.Now;

    int birthYear = dateOfBirth.Year;
    int lastYear = now.Year - 1;

    // obviously, if the user's alive, he/she had a birthday last year;
    // therefore he/she is at least this old
    int minimumAgeInYears = lastYear - birthYear;

    // now the question is: how much time has passed since then?
    double actualAgeInYears = (double)minimumAgeInYears;

    // for every six months that have elapsed since the user's birthday
    // LAST year, add 0.5 to his/her age
    DateTime birthDateLastYear = new DateTime(lastYear, 1, 1)
        .AddDays(dateOfBirth.DayOfYear);

    DateTime comparisonDate = birthDateLastYear
        .AddMonths(6);

    while (comparisonDate < now) {
        actualAgeInYears += 0.5;
        comparisonDate = comparisonDate.AddMonths(6);
    }

    return Convert.ToDecimal(actualAgeInYears);
}

---

This idea that people have been suggesting of checking dateOfBirth.AddMonths(6) is wrong. Since dateOfBirth is a DateTime, it represents the user’s birth date, not their birth day.

What you want to check is if six months have elapsed since the user’s last birthday–not the date they were born. Here’s one way to do that:

DateTime lastBirthDay = GetLastBirthday(dateOfBirth);

if (DateTime.Today > lastBirthDay.AddMonths(6))
{
    adjust = 0.5M;
}
else
{
    adjust = 0M;
}

DateTime GetLastBirthday(DateTime dateOfBirth)
{
    int currentYear = DateTime.Now.Year;
    int birthMonth = dateOfBirth.Month;
    int birthDay = dateOfBirth.Day;

    // if user was born on Feb 29 and this year is NOT a leap year,
    // we'll say the user's birthday this year falls on Feb 28
    if (birthMonth == 2 && birthDay == 29 && !IsLeapYear(currentYear))
        birthDay = 28;

    DateTime birthdayThisYear = new DateTime(
        currentYear,
        birthMonth,
        birthDay
    );

    if (DateTime.Today > birthdayThisYear)
        return birthdayThisYear;
    else
        return birthdayThisYear.AddYears(-1);
}

bool IsLeapYear(int year) {
    return year % 400 == 0 || (year % 4 == 0 && year % 100 != 0);
}