A type for Date only in C# - why is there no Date type?

In our C# project w开发者_如何学运维e have the need for representing a date without a time. I know of the existence of the DateTime, however, it incorporates a time of day as well.

• I want to make explicit that certain variables and method-arguments are date-based.
• Hence I can't use the DateTime.Date property

What are the standard approaches to this problem?

• Why is there no Date class in C#?
• Does anyone have a nice implementation using a struct and maybe some extensionmethods on DateTime and maybe implementing some operators such as == and <, > ?

Allow me to add an update to this classic question:

• DateOnly (and TimeOnly) types have been added to .NET 6, starting with Preview 4. See my other answer here.

• Jon Skeet's Noda Time library is now quite mature, and has a date-only type called LocalDate. (Local in this case just means local to someone, not necessarily local to the computer where the code is running.)

I've studied this problem significantly, so I'll also share several reasons for the necessity of these types:

1. There is a logical discrepancy between a date-only, and a date-at-midnight value.

• Not every local day has a midnight in every time zone. Example: Brazil's spring-forward daylight saving time transition moves the clock from 11:59:59 to 01:00:00.

• A date-time always refers to a specific time within the day, while a date-only may refer to the beginning of the day, the end of the day, or the entire range of the day.

2. Attaching a time to a date can lead to the date changing as the value is passed from one environment to another, if time zones are not watched very carefully. This commonly occurs in JavaScript (whose Date object is really a date+time), but can easily happen in .NET also, or in the serialization as data is passed between JavaScript and .NET.

3. Serializing a DateTime with XML or JSON (and others) will always include the time, even if it's not important. This is very confusing, especially considering things like birth dates and anniversaries, where the time is irrelevant.

4. Architecturally, DateTime is a DDD value-object, but it violates the Single Responsibly Principle in several ways:

• It is designed as a date+time type, but often is used as date-only (ignoring the time), or time-of-day-only (ignoring the date). (TimeSpan is also often used for time-of-day, but that's another topic.)

• The DateTimeKind value attached to the .Kind property splits the single type into three, The Unspecified kind is really the original intent of the structure, and should be used that way. The Utc kind aligns the value specifically with UTC, and the Local kind aligns the value with the environment's local time zone.

  The problem with having a separate flag for kind is that every time you consume a DateTime, you are supposed to check .Kind to decide what behavior to take. The framework methods all do this, but others often forget. This is truly a SRP violation, as the type now has two different reasons to change (the value, and the kind).

• The two of these lead to API usages that compile, but are often nonsensical, or have strange edge cases caused by side effects. Consider:

       // nonsensical, caused by mixing types
       DateTime dt = DateTime.Today - TimeSpan.FromHours(3);  // when on today??
  
       // strange edge cases, caused by impact of Kind
       var london = TimeZoneInfo.FindSystemTimeZoneById("GMT Standard Time");
       var paris = TimeZoneInfo.FindSystemTimeZoneById("Romance Standard Time");
       var dt = new DateTime(2016, 3, 27, 2, 0, 0);  // unspecified kind
       var delta = paris.GetUtcOffset(dt) - london.GetUtcOffset(dt);  // side effect!
       Console.WriteLine(delta.TotalHours); // 0, when should be 1 !!!
  

In summary, while a DateTime can be used for a date-only, it should only do so when when every place that uses it is very careful to ignore the time, and is also very careful not to try to convert to and from UTC or other time zones.

I suspect there is no dedicate pure Date class because you already have DateTime which can handle it. Having Date would lead to duplication and confusion.

If you want the standard approach look at the DateTime.Date property which gives just the date portion of a DateTime with the time value set to 12:00:00 midnight (00:00:00).

I've emailed refsrcfeedback@microsoft.com and that's their answer

Marcos, this is not a good place to ask questions like these. Try http://stackoverflow.com Short answer is that you need a model to represent a point in time, and DateTime does that, it’s the most useful scenario in practice. The fact that humans use two concepts (date and time) to mark points in time is arbitrary and not useful to separate.

Only decouple where it is warranted, don’t do things just for the sake of doing things blindly. Think of it this way: what problem do you have that is solved by splitting DateTime into Date and Time? And what problems will you get that you don’t have now? Hint: if you look at DateTime usages across the .NET framework: http://referencesource.microsoft.com/#mscorlib/system/datetime.cs#df6b1eba7461813b#references You will see that most are being returned from a method. If we didn’t have a single concept like DateTime, you would have to use out parameters or Tuples to return a pair of Date and Time.

HTH, Kirill Osenkov

In my email I'd questioned if it was because DateTime uses TimeZoneInfo to get the time of the machine - in Now propriety. So I'd say it's because "the business rules" are "too coupled", they confimed that to me.

I created a simple Date struct for times when you need a simple date without worrying about time portion, timezones, local vs. utc, etc.

https://github.com/claycephus/csharp-date

System.DateOnly and System.TimeOnly types were recently added to .NET 6, and are available in the daily builds.

They were included with the .NET 6 Preview 4 release.

See https://github.com/dotnet/runtime/issues/49036

They are in the .NET source code here:

• https://github.com/dotnet/runtime/blob/main/src/libraries/System.Private.CoreLib/src/System/DateOnly.cs

• https://github.com/dotnet/runtime/blob/main/src/libraries/System.Private.CoreLib/src/System/TimeOnly.cs

I've blogged about them here.

If you need to run date comparisons then use

yourdatetime.Date;

If you are displaying to the screen use

yourdatetime.ToShortDateString();

Allow me to speculate: Maybe it is because until SQL Server 2008 there was no Date datatype in SQL so it would be hard so store it in SQL server?? And it is after all a Microsoft Product?

Who knows why it's that way. There are lots of bad design decisions in the .NET framework. However, I think this is a pretty minor one. You can always ignore the time part, so even if some code does decide to have a DateTime refer to more than just the date, the code that cares should only ever look at the date part. Alternatively, you could create a new type that represents just a date and use functions in DateTime to do the heavy lifting (calculations).

Why? We can only speculate and it doesn't do much to help solve engineering problems. A good guess is that DateTime contains all the functionality that such a struct would have.

If it really matters to you, just wrap DateTime in your own immutable struct that only exposes the date (or look at the DateTime.Date property).

In addition to Robert's answer you also have the DateTime.ToShortDateString method. Also, if you really wanted a Date object you could always use the Adapter pattern and wrap the DateTime object exposing only what you want (i.e. month, day, year).

There is always the DateTime.Date property which cuts off the time part of the DateTime. Maybe you can encapsulate or wrap DateTime in your own Date type.

And for the question why, well, I guess you'll have to ask Anders Heljsberg.

Yeah, also System.DateTime is sealed. I've seen some folks play games with this by creating a custom class just to get the string value of the time as mentioned by earlier posts, stuff like:

class CustomDate
{
    public DateTime Date { get; set; }
    public bool IsTimeOnly { get; private set; }

    public CustomDate(bool isTimeOnly)
    {
        this.IsTimeOnly = isTimeOnly;
    }

    public string GetValue()
    {
        if (IsTimeOnly)
        {
            return Date.ToShortTimeString();
        }

        else
        {
            return Date.ToString();
        }
    }
}

This is maybe unnecessary, since you could easily just extract GetShortTimeString from a plain old DateTime type without a new class

Because in order to know the date, you have to know the system time (in ticks), which includes the time - so why throw away that information?

DateTime has a Date property if you don't care at all about the time.

If you use the Date or Today properties to get only the date portion from the DateTime object.

DateTime today = DateTime.Today;
DateTime yesterday = DateTime.Now.AddDays(-1).Date;

Then you will get the date component only with the time component set to midnight.