DateTime.Now-DateTimeNow.AddSecond（-1）如何为零

Question

I run below code in a console application:

while (true)
{
    DateTime dt1 = DateTime.Now;
    DateTime dt2 = DateTime.Now.AddSeconds(-1);

    if ((dt1 - dt2).Seconds != 1)
        Console.Write((dt1 - dt2).Seconds.ToString());
}

And I thought (dt1 - dt2).Seconds is always 1 but as I saw it write many 0 in the output !!??

Why does (dt1 - dt2).Seconds become 0 ?

---

Then I try below code:

if ((dt1.Hour * 3600 - dt2.Hour * 3600 + dt1.Minute * 60 - dt2.Minute * 60 + dt1.Second - dt2.Second) != 1)
    Console.Write((dt1.Second - dt2.Second).ToString());

And this new code say may (dt1.Hour * 3600 - dt2.Hour * 3600 + dt1.Minute * 60 - dt2.Minute * 60 + dt1.Second - dt2.Second) is always 1 (However I know in 23:59:59 => 00:00:00 it should write a value)

Answer 1

Because you evaluate your DateTime.Now value again for your dt2 variable, that's why this little difference is completely expected. That means your second DateTime.Now calculated at least 1 Tick and at most 10 million Ticks (which is equal to 1 second) after the first one.

And since DateTime.Second property calculated with InternalTicks and TicksPerSecond , it returns zero when InternalTicks is less than TicksPerSecond value after calcualte remainder with 60 .

public int Second
{
     get
     {
        return (int)((InternalTicks / TicksPerSecond) % 60); 
     }
} 

If you use same DateTime.Now value for both variable, that should be ok.

DateTime now = DateTime.Now;
DateTime dt1 = now;
DateTime dt2 = now.AddSeconds(-1);

Why (dt1.Hour * 3600 - dt2.Hour * 3600 + dt1.Minute * 60 - dt2.Minute * 60 + dt1.Second - dt2.Second) is always 1?

Well , it is not always 1 . Let's assume your first DateTime.Now (which is dt1 ) generates 00:00:00.00000 . And second DateTime.Now generates 00:00:00.00002 (those are theoretical values) and your dt2 will be 23:59:59.002 .

Let's look at the Hours , Minute and Second values;

dt1.Hour = 0;
dt2.Hour = 23;
dt1.Minute = 0;
dt2.Minute = 59;
dt1.Second = 0;
dt2.Second = 59;

And the result of dt1.Hour * 3600 - dt2.Hour * 3600 + dt1.Minute * 60 - dt2.Minute * 60 + dt1.Second - dt2.Second) will be -86399 not 1 .

Answer 2

Try doing this

 DateTime dt1 = DateTime.Now;
 DateTime dt2 = dt1.AddSeconds(-1);

You have initialized 2 different timestamps (which vary by a tiny fraction of time), when you accessed the DateTime.Now .