Add fractions of a month to a Date

Question

I was looking all over the internet for this functionality but couldn't find any acceptable solution Simple question, How can you add 1.5 month to a DateTime, AddMonth accepts only integer as a parameter. and yes i know i can use AddDays, but it brings many other questions of how to calculate proper number of days depends on a moths you are looking at

my own solution is bellow, but for sure it is not the perfect one

public static DateTime AddMonths(DateTime val, double months)
{
    int integer =(int) Math.Truncate(months);
    double fraction = months - integer;

    val = val.AddMonths(integer);

    double days = DateTime.DaysInMonth(val.Year, val.Month) * fraction;

    val = val.AddDays(days);

    return val;
}

Update: This is a business project requirements. One of the forms has Issue Date (DateTime) and Term Period (double) that can be defined as fraction of a month. My question was exactly of how can it be handled properly with my version of the code. I know this code isn't the best, and I specified in my question that it raises lots of questions that you guys listed in your comments. So do you guys have any suggestions of how to handle all the scenarios? do you guys have any better code for the function

Answer 1

I would write this as an extension method with the following logic, which is really similar to yours. In fact I think the only difference is that you calculate the days based on the number of days in the starting month, and I calculate them based on the difference of adding an extra month (at the end):

1. Separate the whole months and the partial month into two variables
2. Add the whole months to the Date
3. To calculate the days:
   1. Add another whole month
   2. Subtract the date in step 2 from the it
   3. Get the total days difference
   4. Multiply the total days by the partial month value captured in step 1
4. Add that many days to the month

Probably makes more sense in code:

public static class Extensions
{
    public static DateTime AddMonths(this DateTime startingValue, double months)
    {
        // Cast to an int to avoid recursing back to this method
        var wholeMonths = (int)Math.Floor(months);
        var partialMonth = months - wholeMonths;

        var result = startingValue.AddMonths(wholeMonths);
        return result.AddDays(Math.Floor(
            result.AddMonths(1).Subtract(result).TotalDays * partialMonth));
    }
}

Usage

static void Main(string[] args)
{
    var today = DateTime.Now;
    var later = today.AddMonths(3.5);

    Console.WriteLine($"Today: {today}");
    Console.WriteLine($"Today plus 3.5 months: {later}");

    Console.Write("\nDone!\nPress any key to exit...");
    Console.ReadKey();
}

Answer 2

Without a full set of business requirements, I think the simplest solution would be to take the generally accepted number of days in a month, which is approximately 30.4167, and use that number for the final calculation.

public static DateTime AddMonths(DateTime val, double months)
{
    // expand out the number of days in a month to a wider value than 30.4167
    const double daysInMonth = 30.41666667;

    double days = months * daysInMonth;
    // could also just use val.AddDays(days);
    TimeSpan ts = TimeSpan.FromDays(days);
    DateTime dt = val + ts;

    return dt;
}

Let's accommodate leap year (per comments) by calculating the total number of days in a year:

public static DateTime AddMonths(DateTime val, double months)
{
    double daysInYear = new DateTime(val.Year, 12, 31).DayOfYear;
    double daysInMonth = daysInYear / 12;

    return val.AddDays(months * daysInMonth);
}