The Darkness Behind DateTime.Now
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by Keyvan Nayyeri
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A very commonly-used and well-known property of the DateTime structure in .NET is Now which simply returns the current date and time on the machine executing the code. Such a property is provided in almost all programming languages as a built-in feature and has many applications. Unfortunately, most of the .NET developers have been misusing this property for years for purposes other than what it’s supposed to serve for. I can outline two possible reasons for this problem:

·       Most of the developers are careless about all the properties and methods provided by built-in types in a language, so they don’t discover all the details about them. In fact, they use something that just solves their problem regardless of the side-effects.

·       The traditional use of similar methods in languages used by .NET developers in the past has left some bad habits for them while the internal working of this property is slightly different from what is available in some other languages.

I have to admit that I was a developer who used to apply this property for wrong purposes in the early years of my .NET development, and may still misuse that for simple codes where I don’t care much about performance, but it’s not something to use in each and every situation.

To clarify the use of the Now property of DateTime structure I have to state that this property is not designed to be used in cases such as when you want to retrieve the ti

me for internal calculations in your program, store a DateTime value in database, or calculate the runtime performance of a piece of code. In contrast, it is designed to be used when you want to display the current date and time on a machine to your users or store such a value in local log files where you want the local time to be used.

Although it’s hard to give a general guideline on good and bad uses of this property, I guess that the abovementioned examples can give you enough hints on when and where to use this property. A good question that you may ask about this property is why we distinguish these cases and consider some uses as bad practices. The answer is laid in the internal implementation of the Now property shown in listing 1.

Listing 1: Internal implementation of DateTime.Now

public static DateTime Now
            DateTime utcNow = DateTime.UtcNow;
            bool isAmbiguousDst = false;
            long ticks = TimeZoneInfo.GetDateTimeNowUtcOffsetFromUtc(utcNow, 
                  out isAmbiguousDst).Ticks;
            long num = utcNow.Ticks + ticks;
            if (num > 3155378975999999999L)
                  return new DateTime(3155378975999999999L, DateTimeKind.Local);
            if (num < 0L)
                  return new DateTime(0L, DateTimeKind.Local);
            return new DateTime(num, DateTimeKind.Local, isAmbiguousDst);

Essentially, this property is converting the current date and time in UTC to the local values and has an extra processing associated with this which is the source of the overhead that I will discuss later.

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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

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