How Does Datetime.Touniversaltime() Work

How does DateTime.ToUniversalTime() work?

There is no implicit timezone attached to a DateTime object. If you run ToUniversalTime() on it, it uses the timezone of the context that the code is running in.

For example, if I create a DateTime from the epoch of 1/1/1970, it gives me the same DateTime object no matter where in the world I am.

If I run ToUniversalTime() on it when I'm running the code in Greenwich, then I get the same time. If I do it while I live in Vancouver, then I get an offset DateTime object of -8 hours.

This is why it's important to store time related information in your database as UTC times when you need to do any kind of date conversion or localization. Consider if your codebase got moved to a server facility in another timezone ;)

Edit: note from Joel's answer - DateTime objects by default are typed as DateTimeKind.Local. If you parse a date and set it as DateTimeKind.Utc, then ToUniversalTime() performs no conversion.

And here's an article on "Best Practices Coding with Date Times", and an article on Converting DateTimes with .Net.

What does .ToUniversalTime() really do on a DateTime instance (C#)?

The value returned by the "ToUniversalTime" method is determined by the "Kind" property of the current DateTime object. The following describes the possible results:

Kind: Utc 

Results: No conversion is performed.

Kind: Local. 

Results: The current DateTime object is converted to UTC.

Kind: Unspecified. 

Results: The current DateTime object is assumed to be local time, and the conversion is performed as if Kind were Local.

The default is Unspecified.

The ToUniversalTime method converts a DateTime value from local time to UTC. To convert the time in a non-local time zone to UTC, use the TimeZoneInfo.ConvertTimeToUtc(DateTime, TimeZoneInfo) method. To convert a time whose offset from UTC is known, use the ToUniversalTime method.

How DateTime.Parse works in case to universal time format

The Parse method sets the Kind property of the date to DateTimeKind.Unspecified as there is no time zone information in the string. The ToUniversalTime method assumes that the time is local and converts it to UTC.

Ref: DateTime.Parse Method

"Generally, the Parse method returns a DateTime object whose Kind
property is DateTimeKind.Unspecified."

Ref: DateTime.ToUniversalTime Method

"Unspecified: The current DateTime object is assumed to be a local time, and the
conversion is performed as if Kind were Local."

What is the difference between DateTime.UtcNow and DateTime.Now.ToUniversalTime()

Actually it's the other way around. The Now property is implemented as:

public static DateTime Now {
  get {
    return UtcNow.ToLocalTime();
  }
}

DateTime.ToUniversalTime() not correct

Not sure if it's the best solution, but this certainly works:

var InputTime = new DateTime(2016, 10, 12, 12, 22, 0, DateTimeKind.Local);
var offset = TimeZone.CurrentTimeZone.GetUtcOffset(DateTime.Now);
var OutputTime = DateTime.SpecifyKind(InputTime - offset, DateTimeKind.Utc);

DateTime.ToUniversalTime() gets ignored in asp.net application

The ToLocalTime and ToUniversalTime methods on the DateTime object are for converting between UTC and the local time zone of the computer where the code is running. They do not belong in an ASP.Net application. Neither does DateTime.Now.

Many servers follow a best practice of having their time zone set to UTC, so in these environments you will see no difference other than the change of the .Kind metadata property.

In general, in an server-side environment such as ASP.Net, you should not depend on the server's local time zone to be anything in particular. Instead, you should use the TimeZoneInfo class to convert between UTC and a specific time zone. For example:

string tzid = "Romance Standard Time"; // for Denmark
TimeZoneInfo tz = TimeZoneInfo.FindSystemTimeZoneById(tzid);
DateTime utc = TimeZoneInfo.ConvertTimeToUtc(localTime, tz);

Also, it's important to understand that time zones and culture are two different things. They are not related whatsoever.

What is the difference between DateTime.ToUniversalTime and TimeZoneInfo.ConvertTimeToUtc

If you run the code on a machine in a different timezone, are your calculations still going to work? This is the reason people store and treat all DateTimes as UTC - it removes any ambiguity. You wouldn't need to store the user's timezone. Any machine, anywhere, can pull a date from the database and convert it to and from local times with ease.

If you're storing times in some other timezone, then you have to pull it out, calculate the offset to the desired timezone, including factoring in daylight savings times and international dateline considerations. In your case, you're also storing extra unnecessary information.

Different time output for ToUniversalTime

The ToUniveralTime method converts from the local time zone where the code is running, to UTC.

Since time zones can change their offsets from UTC at different times of the year, the value can easily be different between two different dates - especially since one date is in the winter, and the other is in the summer, due to daylight saving time.

See also, the DST tag wiki, and "time zone != offset" in the timezone tag wiki.

.ToUniversalTime(), why it works this way?

Because of daylight savings. Local time is as it appears on your clock. UTC is continuous without gaps, without ambiguous points in time.

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