Net DateTime.AddMonths(1) doesn't work the same as Javascript d.setMonth(d.getMonth() + 1)

Question

I have client side date validation that requires one particular Date to be one month from a different date so I use d.setMonth(d.getMonth() + 1) and mostly works just fine.

For end of month issues as in 1/31/2009, it returns 3/3/2009 and that's great - that's how I'd prefer it handle it.

In the code behind, I'm also generating this date but DateTime.AddMonths(1) returns 2/28/2009 so that's no good.

Is there some way around this?

Answer 1

The .NET function is undoubtedly more intelligent. But if you want to dumb it down to behave like Javascript, add 31 days instead...

DateTime.AddDays(31)

Answer 2

From your example of 1/31/2009 being changed to 3/3/2009 it sounds like you just want a way to advance the specified date by the number of days in its respective month. (Adding 31 days to the date if it's in January, 28 if in February during a non-leap year, etc...)

So your code would look something like:

d = d.AddDays(DateTime.DaysInMonth(d.Year, d.Month))

Answer 3

Adding to Josh's answer... Not really sure why you want to do this but if you do want a specific behavior that is not built in, remember that you have the ability to write extensions. This extension adds a method called "AddMonthsJ" to DateTime. This gives the behavior that I think you are looking for and allows for it to be easily used. You could modify the extension to be whatever you are looking for in changing months.

class Program
{
    static void Main(string[] args)
    {
        DateTime myDate = DateTime.Parse("01/31/2009");

        myDate = myDate.AddMonthsJ(1);

        Console.WriteLine(myDate.ToShortDateString());

        Console.ReadLine();
    }
}

public static class Extensions
{
    public static DateTime AddMonthsJ(this DateTime oldDate, int months)
    {
        return (oldDate.AddDays(months * 31));
    }
}