Utc Daylight Savings

UTC is not a time zone, but a time standard that is the basis for civil time and time zones worldwide. This means that no country or territory officially uses UTC as a local time. UTC, GMT and Daylight Saving Time. Neither UTC nor GMT ever change for Daylight Saving Time (DST). However, some of the countries that use GMT switch to different. Offset: UTC is 0 hours ahead Greenwich Mean Time (GMT) and is used in Universal Coordinated Universal Time (UTC) is the world time standard that regulates clocks and time. It is successor to Greenwich Mean Time (GMT). For casual use, UTC is the same as GMT, but is. To make sure both were using UTC time, I modified /etc/default/rcS setting UTC=yes. I installed ntp, set the clock time to an incorrect time in the bios, booted Ubuntu, then verified the clock time was correctly set to UTC time using sudo hwclock -show then also from the bios. Howell Mar 12 '15 at 13:46. From March 11, 2018 to November 3, 2018, it meant UTC-4. And from November 4, 2018 to December 31, 2018, it's UTC-5 again. But it gets worse. From Wikipedia: The Uniform Time Act of 1966 ruled that daylight saving time would run from the last Sunday of April until the last Sunday in October in the United States.

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Definition

Indicates whether a specified date and time falls in the range of daylight saving time for the current TimeZoneInfo object's time zone.

Overloads

IsDaylightSavingTime(DateTimeOffset)

Indicates whether a specified date and time falls in the range of daylight saving time for the time zone of the current TimeZoneInfo object.

IsDaylightSavingTime(DateTime)

Indicates whether a specified date and time falls in the range of daylight saving time for the time zone of the current TimeZoneInfo object.

Utc daylight savings offset

Indicates whether a specified date and time falls in the range of daylight saving time for the time zone of the current TimeZoneInfo object.

Parameters

dateTimeOffset
DateTimeOffset

A date and time value.

Returns

Boolean

true if the dateTimeOffset parameter is a daylight saving time; otherwise, false.

Remarks

The return value of TimeZoneInfo.IsDaylightSavingTime is affected by the relationship between the time zone represented by the TimeZoneInfo object and the Offset property of the dateTimeOffset parameter. If dateTimeOffset does not correspond to the current time zone's offset from Coordinated Universal Time (UTC), the method converts that time to the time in the current time zone. It then determines whether that date and time is a daylight saving time.

If the time zone represented by the TimeZoneInfo object does not support daylight saving time, the method always returns false. To determine whether a time zone supports daylight saving time, retrieve the value of its SupportsDaylightSavingTime property.

See also

Indicates whether a specified date and time falls in the range of daylight saving time for the time zone of the current TimeZoneInfo object.

Parameters

Daylight

Returns

Boolean

true if the dateTime parameter is a daylight saving time; otherwise, false.

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Exceptions

The Kind property of the dateTime value is Local and dateTime is an invalid time.

Examples

The following example defines a method named DisplayDateWithTimeZoneName that uses the TimeZoneInfo.IsDaylightSavingTime method to determine whether to display a time zone's standard time name or daylight saving time name.

Remarks

The return value of TimeZoneInfo.IsDaylightSavingTime is affected by the relationship between the time zone represented by the TimeZoneInfo object and the Kind property of the dateTime parameter, as the following table shows.

TimeZoneInfo objectDateTime.Kind propertyResult
TimeZoneInfo.LocalDateTimeKind.LocalDetermines whether dateTime is daylight saving time.
TimeZoneInfo.LocalDateTimeKind.UtcConverts dateTime from Coordinated Universal Time (UTC) to local time and determines whether it is daylight saving time.
TimeZoneInfo.LocalDateTimeKind.UnspecifiedAssumes that dateTime represents local time and determines whether it is daylight saving time.
TimeZoneInfo.UtcDateTimeKind.Local, DateTimeKind.Unspecified, or DateTimeKind.UtcReturns false (UTC does not support daylight saving time).
Any other TimeZoneInfo object.DateTimeKind.LocalConverts the local time to the equivalent time of the TimeZoneInfo object and then determines whether the latter is daylight saving time.
Any other TimeZoneInfo object.DateTimeKind.UtcConverts UTC to the equivalent time of the TimeZoneInfo object and then determines whether the latter is daylight saving time.
Any other TimeZoneInfo object.DateTimeKind.UnspecifiedDetermines whether dateTime is daylight saving time.

If the time zone represented by the TimeZoneInfo object does not support daylight saving time, the method always returns false. A number of time zones, including Utc, do not observe daylight saving time. To determine whether a time zone supports daylight saving time, retrieve the value of its SupportsDaylightSavingTime property.

If the dateTime parameter specifies an ambiguous time in the current object's time zone, the TimeZoneInfo.IsDaylightSavingTime method interprets dateTime as standard time and returns false if its Kind property is DateTimeKind.Local or DateTimeKind.Unspecified. If the Kind property is DateTimeKind.Utc, this method will select the correct ambiguous time and indicate whether it is a daylight saving time.

Because the TimeZoneInfo.IsDaylightSavingTime(DateTime) method can return false for a date and time that is ambiguous (that is, a date and time that can represent either a standard time or a daylight saving time in a particular time zone), the TimeZoneInfo.IsAmbiguousTime(DateTime) method can be paired with the IsDaylightSavingTime(DateTime) method to determine whether a time may be a daylight saving time. Because an ambiguous time is one that can be both a daylight saving time and a standard time, the IsAmbiguousTime(DateTime) method can be called first to determine whether a date and time may be a daylight saving time. If the method returns false, the IsDaylightSavingTime(DateTime) method can be called to determine whether the DateTime value is a daylight saving time. The following example illustrates this technique.

Utc during daylight savings

If the dateTime parameter specifies an invalid time, the method call throws an ArgumentException if the value of the dateTime parameter's Kind property is DateTimeKind.Local; otherwise, the method returns false.

Call the TimeZoneInfo.IsDaylightSavingTime method to determine whether to use a time zone's StandardName value or its DaylightName value when displaying the time zone name. See the Example section for an illustration.

See also

Utc Time Calculator

Applies to

In this post, I review some reasons why it's really difficult to program correctly when using times, dates, time zones, and daylight saving time, and then I'll give some advice for working with them in Python and Django. Also, I'll go over why I hate daylight saving time (DST).

TIME ZONES

Let's start with some problems with time zones, because they're badenough even before we consider DST, but they'll help us ease into it.

Time Zones Shuffle

Utc daylight savings conversion

Time zones are a human invention, and humans tend to change theirminds, so time zones also change over time.

Many parts of the world struggle with time changes. For example, let's look at the Pacific/Apia time zone, which is the timezone of the independent country of Samoa. Through December 29, 2011,it was -11 hours from Coordinated Universal Time (UTC). From December 31, 2011, Pacific/Apia became+13 hours from UTC.

What happened on December 30, 2011? Well, it neverhappened in Samoa, because December 29, 23:59:59-11:00 is followedimmediately by December 31, 0:00:00+13:00.

DateTimeZoneDateTimeZone
2011-12-2923:59:59UTC-112011-12-3010:59:59UTC
2018-12-3100:00:00UTC+132011-12-3011:00:00UTC

That's an extreme example, but time zones change more often thanyou might think, often due to changes in government or country boundaries.

The bottom line here is that even knowing the time and time zone, it'smeaningless unless you also know the date.

Always Convert to UTC?

As programmers, we're encouraged to avoid issues with time zones by'converting' times to UTC (CoordinatedUniversal Time) as early as possible, and convert to the local timezone only when necessary to display times to humans. But there's a problem with that.

If all you care about is the exact moment in the lifetime of theuniverse when an event happened (or is going to happen), then thatadvice is fine. But for humans, the time zone that they expressed a time in can be important, too.

For example, suppose I'm in North Carolina, in theeastern time zone, but I’m planning an event in Memphis, which is in the central time zone. I go to my calendarprogram and carefully enter the date and '3:00 p.m. CST'.The calendar follows the usual convention and converts my entry to UTCby adding 6 hours, so the time is stored as 9:00 p.m. UTC, or 21:00UTC. If the calendar uses Django, there's not even any extra codeneeded for the conversion, because Django does it automatically.

The next day I look at my calendar to continue working on myevent. The event time has been converted to my local time zone,or eastern time, so the calendar shows the event happening at '4:00p.m.' (instead of the 3:00 p.m. that it should be). The conversion is not useful for me,because I want toplan around other events in the location where the event ishappening, which is using CST, so my local time zone is irrelevant.

The bottom line is that following the advice to always converttimes to UTC results in lost information.We're sometimes better off storing times with their non-UTC time zones. That's why it's kind of annoying that Django always 'converts' local times to UTC before savingto the database, or even before returning them from a form.That means the original timezone is lost unless you go to thetrouble of saving it separately and then converting the time from thedatabase back to that time zone after you get it from thedatabase. I wrote about this before.

By the way, I've been putting 'convert' in scare quotes because talkingabout converting times from one time zone to another carriesan implicit assumption that such converting is simple and losesno information, but as we see, that's not really true.

DAYLIGHT SAVING TIME

Daylight saving time (DST) is even more ofa human invention than time zones.

Time zones are a fairly obvious adaptation to the conflict between howour bodies prefer to be active during the hours when the sun is up,and how we communicate time with people in other parts of the world.Historical changes in time zones across the years are annoying, but sincetime zones are a human invention it's not surprising that we'd tweakthem every now and then.

DST, on the other hand, amounts to changing entire time zones twiceevery year. What does US/eastern time zone mean? I don't know,unless you tell me the date. From January 1, 2018 to March 10, 2018, itmeant UTC-5. From March 11, 2018 to November 3, 2018, it meant UTC-4.And from November 4, 2018 to December 31, 2018, it's UTC-5 again.

But it gets worse. FromWikipedia:

The Uniform Time Act of 1966 ruled that daylight saving timewould run from the last Sunday of April until the last Sundayin October in the United States. The act was amended to makethe first Sunday in April the beginning of daylight savingtime as of 1987. The Energy Policy Act of 2005 extendeddaylight saving time in the United States beginning in 2007.So local times change at 2:00 a.m. EST to 3:00 a.m. EDT onthe second Sunday in March and return at 2:00 a.m. EDT to1:00 a.m. EST on the first Sunday in November.

So in a little over 50 years, the rules changed 3 times.

Even if you have complete and accurate information about the rules,daylight saving time complicates things in surprising ways. Forexample, you can't convert 2:30 a.m. March 11, 2018. in US/easterntime zone to UTC, because that time never happened — our clocks had tojump directly from 1:59:59 a.m. to 3:00:00 a.m. See below:

DateTimeZoneDateTimeZone
2018-03-111:59:59EST2018-03-116:59:59UTC
2018-03-113:00:00EDT2018-03-117:00:00UTC

You can't convert 1:30 a.m. November 4, 2018, in US/eastern timezone to UTC either, because that time happened twice. You would haveto specify whether it was 1:30 a.m. November 4, 2018 EDT or 1:30 a.m.November 4, 2018 EST:

DateTimeZoneDateTimeZone
2018-11-041:00:00EDT2018-11-045:00:00UTC
2018-11-041:30:00EDT2018-11-045:30:00UTC
2018-11-041:59:59EDT2018-11-045:59:59UTC
2018-11-041:00:00EST2018-11-046:00:00UTC
2018-11-041:30:00EST2018-11-046:30:00UTC
2018-11-041:59:59EST2018-11-046:59:59UTC

Advice on How to Properly Manage datetimes

Here are some rules I try to follow.

When working in Python, never use naive datetimes. (Those aredatetime objects without timezone information, which unfortunately arethe default in Python, even in Python 3.)

Use the pytz library whenconstructing datetimes, and review the documentationfrequently. Properly managing datetimes is not always intuitive, andusing pytz doesn't prevent me from using it incorrectly anddoing things that will provide the wrong results only for some inputs, making itreally hard to spot bugs. I have to triple-check that I'm following thedocs when I write the code and not rely on testing to find problems.

Let me strengthen that even further. It isnot possibletocorrectly construct datetimes with timezone information usingonly Python's own libraries when dealing with timezones thatuse DST. I must use pytz or something equivalent.

If I'm tempted to use datetime.replace, I need to stop, thinkhard, and find another way to do it. datetime.replace is almostalways the wrong approach, because changing one part of a datetime withoutconsideration of the other parts is almost guaranteed to not do what I expectfor some datetimes.

When using Django, be sure USE_TZ = True.If Django emits warnings about naive datetimes being saved in thedatabase, treat them as if they were fatal errors, track them down,and fix them. If I want to, I can even turn them into actual fatalerrors; see this Django documentation.

When processing user input, consider whether a datetime's originaltimezone needs to be preserved, or if it's okay to just store thedatetime as UTC. If the original timezone is important, see this post I wrote about how to get and store it.

Conclusion

Utc Daylight Savings 2020

Working with human times correctly is complicated, unintuitive,and needs a lot of careful attention to detail to get right. Further, some of the oft-given advice, like always working in UTC, can cause problems of its own.