How to properly convert DateofBirth to age inside a viewmodal

Question

I'm trying to convert a personnel's date of birth to their actual age within a ViewModel inside of another viewmodel I plan on calling in the front end. I've managed to create public DateTime? PersonnelDOB { get; set; } and it's bringing back their Date of Birth I.E 6/12/1972 I need to convert this to their actual age so instead of 6/12/1972, it'll be "48 years old"

the issue to this current problem is that 'dob' is a DateTime and 'today.year' is an int. I can't subtract a DateTime from an int. I need to also make sure I account for leapyears and for it to actually accurately output their age. I also will want to check that dob isn't null. I dont have to do this within a viewmodel I created, it was just an avenue I was exploring. Thank you all for your help!

public DateTime? PersonnelDOB { get; set; }
    
    public PersonnelDOBViewModel()
    {
        var dob = PersonnelDOB;

        // Save today's date.
        var today = DateTime.Today;

        // Calculate the age.
        var age = today.Year - dob;

        // Go back to the year the person was born in case of a leap year
        if (dob > today.AddYears(-age)) age--;
        return age;
    }

** A coworker helped me out and for those of you interested in the right answer - here it is

public DateTime? PersonnelDOB { get; set; }

    public int? PersonnelAge
    {
        get
        {
            if (!PersonnelDOB.HasValue)
                return null;
            
            DateTime today = DateTime.Today;
            
            int years = today.Year - PersonnelDOB.Value.Year;

            years -= (today.Month < PersonnelDOB.Value.Month || (today.Month == PersonnelDOB.Value.Month && today.Day < PersonnelDOB.Value.Day)) ? 1 : 0;
            return years;
        }
    }

Answer 1

If it were me, I'd lean on NodaTime for this since you can calculate the Period between the date-of-birth and "today".

Per NodaTime's documentation:

A Period is a number of years, months, weeks, days, hours and so on, which can be added to (or subtracted from) a LocalDateTime, LocalDate or LocalTime. The amount of elapsed time represented by a Period isn't fixed: a period of "one month" is effectively longer when added to January 1st than when added to February 1st, because February is always shorter than January.

There's even a "recipe" for what you're trying to do within the documentation.

Adapted to your view model, it could look something like this:

public class PersonnelDOBViewModel
{
    private readonly ZonedClock _clock;

    public PersonnelDOBViewModel()
    {
        // Depending on your goals, you may want to use a user-defined
        // time zone here
        var timezone = DateTimeZoneProviders.Tzdb.GetSystemDefault();

        _clock = SystemClock.Instance.InZone(timezone);
    }

    public DateTime? PersonnelDOB { get; set; }

    public int? Age
    {
        get
        {
            if (PersonnelDOB == null)
            {
                return null;
            }

            var dob = LocalDate.FromDateTime(PersonnelDOB.Value);
            var today = _clock.GetCurrentDate();

            return Period.Between(dob, today).Years;
        }
    }
}

Answer 2

Try using DateTime.Now (or DateTime.UtcNow) instead of DateTime.Today.Year.
Also keep in mind that age calculations get tricky when you take time zone into account.

var age = dob.HasValue ? GetAge(dob.Value, DateTime.Now) : (int?)null;

private int GetAge(DateTime dob, DateTime currentTime)
{
    var years = currentTime.Year - dob.Year;

    if (currentTime.Month < dob.Month || (currentTime.Month == dob.Month && currentTime.Day < dob.Day))
    {
        years--;
    }
    
    return years;
}