Why does TimeSpan not have a Years property?

Question

I was writing a converter that takes a person's date of birth and produces their age in years. I wrote something that looked like this:

public class DateOfBirthToAgeConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        var date = value as DateTime?;
        if (date == null) return null;
        return (DateTime.Now - date).Years;
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

I found that there is no Years property on the TimeSpan that results from the subtraction of two DateTime objects. I was somewhat surprised by this. I thought about why there might not be a Years. I figured that it might be because of the leap day, but by that logic, there shouldn't be Days because of daylight savings.

The absence of Months made sense, since there is no standard month length.

I was able to write some different code to get the correct age, but I still really want to know why there is no Years or Weeks property on TimeSpan. Does anyone know the reason?

Answer 1

A TimeSpan only contains the difference between two DateTime values. It is unknown which year this TimeSpan is in. That's also why it doesn't have a Months property.

Example:

TimeSpan.FromDays(60)

How many months are that? 1 or 2?

---

The absence of Months made sense since there is no standard month length.

There is no standard year length either because of leap years.

Workaround: If you really want to display an approximate value, then doing TimeSpan.TotalDays / 365.2425 will do just fine.

Edit: But only for rough estimations and not for birthdays. In birthday calculation, leap days will accumulate every 4 years as pointed out by Henk Holterman in the comments. Take a look here for calculation of birthdays.

Answer 2

Programmer's life is really hard.

The length of year is variable. Some years have 365 days and some have 366 days. According to the calendar, some years could even have missing days. If talking about culture it becomes more difficult since Chinese lunar calendar can have 13 months a year.

The length of month is variable, and this is well-known. This is also to know that in other calendars things can get worse.

The length of day is variable, because of daylight savings and this is not just culture dependent but also geography dependent.

The length of hour and minute are variable, because of leap seconds.

It seems the only thing that is reliable is the length of a second. So internally, timespan is stored in seconds (or milliseconds, which is the same).

But the variability of time units makes the answer "how many (years/months/days/hours/minites) for n seconds?" being always inaccurate.

This is why the developers end up with a solution that is useful in practical but not precise. They simply ignore daylight savings and leap seconds. However, since people hardly ask about years and months, they just decided not to answer those questions.

Answer 3

Rhetorical question: Without a point of reference, how long is a year?

Because a TimeSpan does not have a fixed point in time, it is not possible to unambiguously say how long a year at an unknown time will be. In the simplest case, it might be 365 or 366 days. There are considerably more cases that would affect the outcome.

Answer 4

I figured that it might be because of the leap day, but by that logic, there shouldn't be Days because of daylight savings.

You have a point there; subtracting two dates doesn't handle daylight savings ideally. If the dates are local time, you may get an unexpected result.

A change in daylight savings time means a gap or overlap in the local time, and that is ignored if you do calculations with the dates. So, if you want to get the exact difference between two DateTime values that are local time, you should convert them to UTC first as that has linear time:

TimeSpan diff = date1.ToUniversalTime() - date2.ToUniversalTime();

The reason that the TimeSpan doesn't have years is that years differ in length. The daylight savings issue is an effect of how you calculate the TimeSpan and can be circumvented, but there is no "linear years" that you can use to circumvent leap years.

Answer 5

Timespan simply stores number of milliseconds. If you have (1000 * 60 * 60 * 24 * 365.5) 365.5 days worth of milliseconds it's impossible to know if that number of milliseconds spans an entire year and into the next, if it's just short of a year, or if it spans across three years. Same with 30.5 days worth of milliseconds, could span into a second month, could be less than a month, could span across three months.

Answer 6

Assuming you can start with 2 dates and not a single TimeSpan, then you can get the difference in years like this...

The DateTime.AddYears() function has already solved the majority of issues you may encounter with leap years etc. So for a difference in years, start with the earliest of the two dates and repeatedly add 1 year using .AddYears(1) and increment a counter till you have a date which exceeds the later date of the two and the difference in years will be the value of the counter -1.

If you think the 2 dates may be thousands of years apart, you could adapt this logic to repeatedly +100 years, when the later date is exceeded, repeatedly -10 years, when you go below the higher date, finally +1 again till the higher date is exceeded. If you + and - from the counter the number of years you are adding and taking away, you'll end up with counter -1 being the number of whole years difference again.

public int AgeInYears(DateTime date1, DateTime date2)
{
    int age = 0;
    DateTime low;
    DateTime high;
    DateTime test;
    if(date1 < date2)
    {
        low = date1;
        high = date2;
    }
    else
    {
        low = date2;
        high = date1;
    }
    
    test = low;
    while(test < high)
    {
        test = test.AddYears(100);
        age += 100;
    }
    
    while (test > high)
    {
        test = test.AddYears(-10);
        age -= 10;
    }
    
    while(test <= high)
    {
        test = test.AddYears(1);
        age++;
    }

    return age - 1;
}