Difference between System.DateTime.Now and System.DateTime.Today
DateTime.Now returns a DateTime value that consists of the local date and time of the computer where the code is running. It has DateTimeKind.Local assigned to its Kind property. It is equivalent to calling any of the following:
DateTime.UtcNow.ToLocalTime()DateTimeOffset.UtcNow.LocalDateTimeDateTimeOffset.Now.LocalDateTimeTimeZoneInfo.ConvertTime(DateTime.UtcNow, TimeZoneInfo.Local)TimeZoneInfo.ConvertTimeFromUtc(DateTime.UtcNow, TimeZoneInfo.Local)
DateTime.Today returns a DateTime value that has the same year, month, and day components as any of the above expressions, but with the time components set to zero. It also has DateTimeKind.Local in its Kind property. It is equivalent to any of the following:
DateTime.Now.DateDateTime.UtcNow.ToLocalTime().DateDateTimeOffset.UtcNow.LocalDateTime.DateDateTimeOffset.Now.LocalDateTime.DateTimeZoneInfo.ConvertTime(DateTime.UtcNow, TimeZoneInfo.Local).DateTimeZoneInfo.ConvertTimeFromUtc(DateTime.UtcNow, TimeZoneInfo.Local).Date
Note that internally, the system clock is in terms of UTC, so when you call DateTime.Now it first gets the UTC time (via the GetSystemTimeAsFileTime function in the Win32 API) and then it converts the value to the local time zone. (Therefore DateTime.Now.ToUniversalTime() is more expensive than DateTime.UtcNow.)
Also note that DateTimeOffset.Now.DateTime will have similar values to DateTime.Now, but it will have DateTimeKind.Unspecified rather than DateTimeKind.Local - which could lead to other errors depending on what you do with it.
So, the simple answer is that DateTime.Today is equivalent to DateTime.Now.Date.
But IMHO - You shouldn't use either one of these, or any of the above equivalents.
When you ask for DateTime.Now, you are asking for the value of the local calendar clock of the computer that the code is running on. But what you get back does not have any information about that clock! The best that you get is that DateTime.Now.Kind == DateTimeKind.Local. But whose local is it? That information gets lost as soon as you do anything with the value, such as store it in a database, display it on screen, or transmit it using a web service.
If your local time zone follows any daylight savings rules, you do not get that information back from DateTime.Now. In ambiguous times, such as during a "fall-back" transition, you won't know which of the two possible moments correspond to the value you retrieved with DateTime.Now. For example, say your system time zone is set to Mountain Time (US & Canada) and you ask for DateTime.Now in the early hours of November 3rd, 2013. What does the result 2013-11-03 01:00:00 mean? There are two moments of instantaneous time represented by this same calendar datetime. If I were to send this value to someone else, they would have no idea which one I meant. Especially if they are in a time zone where the rules are different.
The best thing you could do would be to use DateTimeOffset instead:
// This will always be unambiguous.
DateTimeOffset now = DateTimeOffset.Now;
Now for the same scenario I described above, I get the value 2013-11-03 01:00:00 -0600 before the transition, or 2013-11-03 01:00:00 -0700 after the transition. Anyone looking at these values can tell what I meant.
I wrote a blog post on this very subject. Please read - The Case Against DateTime.Now.
Also, there are some places in this world (such as Brazil) where the "spring-forward" transition happens exactly at Midnight. The clocks go from 23:59 to 01:00. This means that the value you get for DateTime.Today on that date, does not exist! Even if you use DateTimeOffset.Now.Date, you are getting the same result, and you still have this problem. It is because traditionally, there has been no such thing as a Date object in .Net. So regardless of how you obtain the value, once you strip off the time - you have to remember that it doesn't really represent "midnight", even though that's the value you're working with.
If you really want a fully correct solution to this problem, the best approach is to use NodaTime. The LocalDate class properly represents a date without a time. You can get the current date for any time zone, including the local system time zone:
using NodaTime;
...
Instant now = SystemClock.Instance.Now;
DateTimeZone zone1 = DateTimeZoneProviders.Tzdb.GetSystemDefault();
LocalDate todayInTheSystemZone = now.InZone(zone1).Date;
DateTimeZone zone2 = DateTimeZoneProviders.Tzdb["America/New_York"];
LocalDate todayInTheOtherZone = now.InZone(zone2).Date;
If you don't want to use Noda Time, there is now another option. I've contributed an implementation of a date-only object to the .Net CoreFX Lab project. You can find the System.Time package object in their MyGet feed. Once added to your project, you will find you can do any of the following:
using System;
...
Date localDate = Date.Today;
Date utcDate = Date.UtcToday;
Date tzSpecificDate = Date.TodayInTimeZone(anyTimeZoneInfoObject);
Time. .Now includes the 09:23:12 or whatever; .Today is the date-part only (at 00:00:00 on that day).
So use .Now if you want to include the time, and .Today if you just want the date!
.Today is essentially the same as .Now.Date
The DateTime.Now property returns the current date and time, for example 2011-07-01 10:09.45310.
The DateTime.Today property returns the current date with the time compnents set to zero, for example 2011-07-01 00:00.00000.
The DateTime.Today property actually is implemented to return DateTime.Now.Date:
public static DateTime Today {
get {
DateTime now = DateTime.Now;
return now.Date;
}
}
DateTime.Today represents the current system date with the time part set to 00:00:00
and
DateTime.Now represents the current system date and time
I thought of Adding these links -
- A brief History of DateTime - By Anthony Moore by BCL team
- Choosing between Datetime and DateTime Offset - by MSDN
- Do not forget SQL server 2008 onwards has a new Datatype as DateTimeOffset
- The .NET Framework includes the DateTime, DateTimeOffset, and TimeZoneInfo types, all of which can be used to build applications that work with dates and times.
- Performing Arithmetic Operations with Dates and Times-MSDN
Coming back to original question , Using Reflector i have explained the difference in code
public static DateTime Today
{
get
{
return DateTime.Now.Date; // It returns the date part of Now
//Date Property
// returns same date as this instance, and the time value set to 12:00:00 midnight (00:00:00)
}
}
private const long TicksPerMillisecond = 10000L;
private const long TicksPerDay = 864000000000L;
private const int MillisPerDay = 86400000;
public DateTime Date
{
get
{
long internalTicks = this.InternalTicks; // Date this instance is converted to Ticks
return new DateTime((ulong) (internalTicks - internalTicks % 864000000000L) | this.InternalKind);
// Modulo of TicksPerDay is subtracted - which brings the time to Midnight time
}
}
public static DateTime Now
{
get
{
/* this is why I guess Jon Skeet is recommending to use UtcNow as you can see in one of the above comment*/
DateTime utcNow = DateTime.UtcNow;
/* After this i guess it is Timezone conversion */
bool isAmbiguousLocalDst = false;
long ticks1 = TimeZoneInfo.GetDateTimeNowUtcOffsetFromUtc(utcNow, out isAmbiguousLocalDst).Ticks;
long ticks2 = utcNow.Ticks + ticks1;
if (ticks2 > 3155378975999999999L)
return new DateTime(3155378975999999999L, DateTimeKind.Local);
if (ticks2 < 0L)
return new DateTime(0L, DateTimeKind.Local);
else
return new DateTime(ticks2, DateTimeKind.Local, isAmbiguousLocalDst);
}
}