Adding decimal years to a DateTime

Question

When I encountered this I thought it would be a trivial challenge involved TimeSpans and basic DateTime arithmetic. Unless I have missed some really obvious I was wrong...

How would you add 13.245 years to 3/22/2023 5:25:00 AM?

The closest I get is this snippet:

long ticks = (long)((365.0M - 4) * (decimal)TimeSpan.TicksPerDay * 13.245M); 
            
DateTime futureDate= new DateTime(2023, 3, 22, 5, 25, 0).AddTicks(ticks);
Console.WriteLine(futureDate.ToString());

Which gives me an output of 4/23/2036 4:05:48 PM that I am not entirely confident in. Also, notice the way I have had to manually handle leaps years with:

365.0M - 4

Answer 1

Your approach is almost correct, but there are a few issues to consider:

Leap years: As you mentioned, you need to account for leap years since they have an extra day. One way to do this is to calculate the number of days between the two dates and then add the fraction of a day corresponding to the number of leap years in that period.

Precision: The value of "13.245" years is not exact since a year is not exactly 365 days. It's closer to 365.2425 days. This may not matter for small time intervals, but for longer periods it can make a significant difference.

Time zone: The code you provided assumes that the input date is in the local time zone, which may not be what you intended. If you want to work with UTC dates, you should use the DateTimeOffset structure instead of DateTime.

Here's a modified version of your code that takes these issues into account:

decimal yearsToAdd = 13.245M;
DateTime inputDate = new DateTime(2023, 3, 22, 5, 25, 0);
int inputYear = inputDate.Year;

// Calculate the number of leap years between the input year and the future year
int futureYear = inputYear + (int)yearsToAdd;
int numLeapYears = Enumerable.Range(inputYear, futureYear - inputYear)
    .Count(y => DateTime.IsLeapYear(y));

// Calculate the number of days to add, including leap years
TimeSpan daysToAdd = TimeSpan.FromDays(yearsToAdd * 365.2425M + numLeapYears);

// Add the days to the input date
DateTime futureDate = inputDate.Add(daysToAdd);

Console.WriteLine(futureDate.ToString());

This code calculates the number of leap years between the input year and the future year using LINQ's Enumerable.Range method. It then calculates the total number of days to add, including the fraction of a day corresponding to the partial year and the leap years. Finally, it adds the resulting TimeSpan to the input date to obtain the future date.

Answer 2

Personally I would first add the whole years (13), than check how much ticks the year have and add the decimals depending on the ticks. Something like this:

private static DateTime AddYears(DateTime date, double years)
{
    date = date.AddYears((int)years);
    double remainingYearsToAdd = years - (int)years;

    long ticksInThisYear = new DateTime(date.Year + 1, 1, 1).Ticks - new DateTime(date.Year, 1, 1).Ticks;
    long elapsedTicksThisYear = date.Ticks - new DateTime(date.Year, 1, 1).Ticks;
    double elapsedPercentageThisYear = 1d / ticksInThisYear * elapsedTicksThisYear;
    if (elapsedPercentageThisYear + remainingYearsToAdd <= 1)
    {
        return date.AddTicks((long)(remainingYearsToAdd * ticksInThisYear));
    }
    else
    {
        remainingYearsToAdd -= elapsedTicksThisYear / (double)ticksInThisYear;
        date = new DateTime(date.Year + 1, 1, 1);
        long ticksInNextYear = new DateTime(date.Year + 1, 1, 1).Ticks - new DateTime(date.Year, 1, 1).Ticks;
        return date.AddTicks((long)(remainingYearsToAdd * ticksInNextYear));
    }
}

Answer 3

I see this:

How would you add 13.245 years to 3/22/2023 5:25:00 AM?
...gives me an output of 4/23/2036 4:05:48 PM that I am not entirely confident in.

That's clearly not accurate. With a starting date in March and fractional year portion of 0.25, you should expect to end up in June, not April.

So let's try this instead:

private static DateTime AddYears(DateTime startDate, double years)
{
    startDate = startDate.AddYears((int)years);
    double remainder = years - (int)years;
    double yearSeconds = (new DateTime(startDate.Year + 1, 1, 1) - new DateTime(startDate.Year, 1, 1)).TotalSeconds;

    return startDate.AddSeconds(yearSeconds * remainder);
}

Using the built-in date math functions helps a lot, and going down to the second has the advantage of accepting a double for the final addition, and allowing greater precision. Now we get a result date of 06/21/2036 5:25:00 PM. This sounds more like it. We're not exactly 3 months later in the year (that would be 6/22/2036 5:25:00 AM, so we "lost" 12 hours), but not all months are the same, so this looks reasonably accurate.

However, there's still potential for error because the remainder could put us into a new year which has a different length: possible change in the leap year or other things like the odd leap second. For example, say the starting date is 2023-12-31 23:23:59, and the addition is 0.99. The code above assumes the year length based on that initial 365 day year (0 whole years to add), but nearly all of the final fractional addition takes place the next year, which has 366 days. You'll end up nearly a whole day short of where you expect.

To get more accurate, we want to add the fractional part only up to the end of the year, and then recalculate whatever is left based on the new year.

private static DateTime AddYears(this DateTime startDate, double years)
{
    startDate= startDate.AddYears((int)years);
    double remainder = years - (int)years;
    double yearSeconds = (new DateTime(startDate.Year + 1, 1, 1) - new DateTime(startDate.Year, 1, 1)).TotalSeconds;

    var result = startDate.AddSeconds(yearSeconds * remainder);
    if (result.Year == startDate.Year) return result;

    // we crossed into a near year, so need to recalculate fractional portion from within the ending year
    result = new DateTime(result.Year, 1, 1);

    // how much of the partial year did we already use?
    double usedFraction = (result - startDate).TotalSeconds;
    usedFraction = (usedFraction/yearSeconds);

    // how much of the partial year is still to add in the new year?
    remainder = remainder - usedFraction;

    //seconds in target year:
     yearSeconds = (new DateTime(result.Year + 1, 1, 1) - result).TotalSeconds;
   
    return result.AddSeconds(yearSeconds * remainder);     
}

Knowing Jon Skeet was looking at this question, I wouldn't be surprised if Noda Time makes this easier. I'm sure there are other potentials for error, as well (at least fractional seconds around the end/start of the year boundary), but I feel like this would put you close enough for most purposes.