为什么 DateTime.AddHours 似乎不起作用？

Question

I have same result 1338161400 when I do

    DateTime origin = new DateTime(1970, 1, 1, 0, 0, 0, 0);
    DateTime date = DateTime.Parse(@"28/05/12 01:30");
    TimeSpan diff = date.ToUniversalTime() - origin;
    Console.WriteLine( (Math.Floor(diff.TotalSeconds)).ToString());

as well as when I use date.AddHours(-4) :

    DateTime origin = new DateTime(1970, 1, 1, 0, 0, 0, 0);
    DateTime date = DateTime.Parse(@"28/05/12 01:30");
    date.AddHours(-4);
    TimeSpan diff = date.ToUniversalTime() - origin;
    Console.WriteLine( (Math.Floor(diff.TotalSeconds)).ToString());

I try to get 1338168600 like http://www.mbari.org/staff/rich/utccalc.htm

Update:

Thanks I changed to

    DateTime origin = new DateTime(1970, 1, 1, 0, 0, 0, 0);
    DateTime date = DateTime.Parse(@"28/05/2012 01:30");
    date = date.AddHours(-4);
    date = date.ToUniversalTime();
    TimeSpan diff = date - origin;
    Console.WriteLine((Math.Floor(diff.TotalSeconds)).ToString());  

But I got 1338147000 still not 1338168600

Answer 1

Dates are immutable objects, ie they cannot be modified after creation. DateTime.AddHours returns a new DateTime instance which is shifted backwards by 4 hours but "date" will not be modified.

Use:

DateTime newDate = date.AddHours(-4);

Answer 2

AddHours returns a new DateTime object which is the result of adding the hours onto the original. The original is left unchanged.

Thus you want date = date.AddHours(-4); instead of just date.AddHours(-4);

Answer 3

The original date is set 4 hours backward but you didn't set it to new date. try this instead:

date = date.AddHours(-4);

Answer 4

AddHours只返回新的递增日期，使用：

date = date.AddHours(-4);

Answer 5

DateTime is not immutable. It's just that the DateTime methods don't change the struct, but instead return a new one.

If you do this:

DateTime d1 = DateTime.Now;
DateTime d2 = DateTime.Now;
d1 = d2;

Then, the struct d1 will be overwritten with d2's values.

I don't know if it's possible to have an immutable struct.

Answer 6

AddHours returns new DateTime object. do this:

date = date.AddHours(-4);

Documentation: link

Answer 7

Try if this works:

System.DateTime today = System.DateTime.Now;
System.TimeSpan duration = new System.TimeSpan(0, 11, 0, 0);
System.DateTime answer = today.Add(duration);
System.Console.WriteLine("{0:dddd}", answer);

This seems to change the day in the date also on adding hours.

Answer 8

This is my solution:

     private DateTime SetTime(DateTime datetime, string span="max")
     {
        var hour = datetime.Hour;
        var min = datetime.Minute;
        var sec = datetime.Second;
        if (span == "min")
            datetime = datetime.Add(new TimeSpan(-hour, -min, -sec));
        else
            datetime = datetime.Add(new TimeSpan(23-hour, 59-min, 59-sec));
        return datetime;
     }

     public DateTime GetDateTimes()
     {
            var start = "1/1/2022";
            var end = "31/3/2022";
            var cultureInfo = new CultureInfo("es-ES");
            var startDate = DateTime.Parse(start, cultureInfo);
            var endDate = DateTime.Parse(end, cultureInfo);
            if (startDate > endDate)
                (startDate, endDate) = (endDate, startDate);
            startDate = SetTime(startDate, "min");
            endDate = SetTime(endDate);
            Console.Writeline($"start: {startDate} - end: {endDate}");
            // returns: start: 01/01/2022 00:00:00 - end: 31/03/2022 23:59:59
    }

Method setTime makes that you want.

GetDateTimes method prevents for swapping start (older date) and end (newer date).

(I'm going from this question .)