The Darkness Behind DateTime.Now
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by Keyvan Nayyeri
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Comparison

Nothing is better than seeing some numbers reflecting the points mentioned above, so here I try to write simple programs that compare the performance of DateTime.Now, DateTime.UtcNow, and Stopwatch. Normally, it’s hard to compare the properties of DateTime structure with Stopwatch as they are different by nature, however, thinking a little bit about good examples, it’s possible to relate them together and compare their runtime performance.

Here I develop three pieces of code that accomplish the same goal using these three approaches. I generate different sample sizes (incremented by a unit of 500 for 10 sample data batches) and perform a very basic (and senseless) task. I use DateTime.Now, DateTime.UtcNow, and Stopwatch to calculate the time that it takes to run my code. I measure my elapsed time using QueryPerformanceCounter to have a good precision.

Listing 5: Code to test DateTime.Now

private static void TestDateTimeNow()
{
    Console.WriteLine("Testing DateTime.Now ...");
 
    for (int sampleSize = 500; sampleSize <= 5000; sampleSize += 500)
    {
        long start = 0;
        QueryPerformanceCounter(ref start);
 
        for (int counter = 0; counter < sampleSize; counter++)
        {
            DateTime startTime = DateTime.Now;
 
                int dumbSum = 0;
                for (int temp = 0; temp < 5; temp++)
                    dumbSum++;
 
            DateTime endTime = DateTime.Now;
 
            TimeSpan duration = endTime - startTime;
        }
 
        long end = 0;
        QueryPerformanceCounter(ref end);
            
        long time = 0;
        time = end - start;
 
        Console.WriteLine("Sample Size: {0} - Time: {1}", sampleSize, time);
    }
}

A very similar code can be used with DateTime.UtcNow (listing 6).

Listing 6: Code to test DateTime.Now

private static void TestDateTimeUtcNow()
{
    Console.WriteLine("Testing DateTime.UtcNow ...");
 
    for (int sampleSize = 500; sampleSize <= 5000; sampleSize += 500)
    {
        long start = 0;
        QueryPerformanceCounter(ref start);
 
        for (int counter = 0; counter < sampleSize; counter++)
        {
            DateTime startTime = DateTime.UtcNow;
 
                int dumbSum = 0;
                for (int temp = 0; temp < 5; temp++)
                    dumbSum++;
 
            DateTime endTime = DateTime.UtcNow;
 
            TimeSpan duration = endTime - startTime;
        }
 
        long end = 0;
        QueryPerformanceCounter(ref end);
            
        long time = 0;
        time = end - start;
 
        Console.WriteLine("Sample Size: {0} - Time: {1}", sampleSize, time);
    }
}

And finally, Stopwatch can be applied to measure the duration of time to execute this code using its Start and Stop methods as well as its Elapsed property (listing 7).

Listing 7: Code to test Stopwatch

private static void TestStopwatch()
{
    Console.WriteLine("Testing Stopwatch ...");
 
    for (int sampleSize = 500; sampleSize <= 5000; sampleSize += 500)
    {
        long start = 0;
        QueryPerformanceCounter(ref start);
 
        for (int counter = 0; counter < sampleSize; counter++)
        {
            Stopwatch watch = new Stopwatch();
 
            watch.Start();
 
            int dumbSum = 0;
            for (int temp = 0; temp < 5; temp++)
                dumbSum++;
 
            watch.Stop();
 
            TimeSpan duration = watch.Elapsed;
        }
 
        long end = 0;
        QueryPerformanceCounter(ref end);
 
        long time = 0;
        time = end - start;
 
        Console.WriteLine("Sample Size: {0} - Time: {1}", sampleSize, time);
    }
}

Sometimes a picture is worth a thousand words and figure 1 can reflect whatever I’ve mentioned so far.

Figure 1: Quantitative comparison between DateTime.Now, DateTime.UtcNow, and Stopwatch

Here we see some interesting results and it’s more interesting when we consider the fact that these codes are tested with realistic sample data sizes between 500 and 5000. Here DateTime.Now (the blue line) has the worst performance at top followed by Stopwatch (the green line) and DateTime.UtcNow (the red line). UtcNow stands much lower than the other options with a very steady pace (unlike other two options) and surprisingly, performs better than Stopwatch. This is of course expected as I showed you the internal implementation of Stopwatch which is using DateTime.UtcNow with some extra processing.

Here a good question is why we need to use Stopwatch when DateTime.UtcNow is performing better. The answer is that it’s possible to do that and actually, it’s much better if you’re sure that you get such a significant difference by applying UtcNow rather than Stopwatch. But there is a major and a minor advantage for Stopwatch over UtcNow. The major advantage is that Stopwatch can perform in a higher resolution by applying QueryPerformanceCounter rather than DateTime.UtcNow. The minor advantage is that Stopwatch provides a more fluent and human-readable method to perform this task.


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User Comments

Title: 你们不能明白   
Name: 巴拿马
Date: 2012-12-08 3:41:36 AM
Comment:
巴拿马






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