Daylight saving time and time zone best practices

Question

I am hoping to make this question and the answers to it the definitive guide to dealing with daylight saving time  in particular for dealing with the actual change overs    If you have anything to add  please do  Many systems are dependent on keeping accurate time  the problem is with changes to time due to daylight savings - moving the clock forward or backwards   For instance  one has business rules in an order taking system that depend on the time of the order - if the clock changes  the rules might not be as clear  How should the time of the order be persisted  There are of course an endless number of scenarios - this one is simply an illustrative one    How have you dealt with the daylight saving issue   What assumptions are part of your solution   looking for context here    As important  if not more so    What did you try that did not work   Why did it not work    I would be interested in programming  OS  data persistence and other pertinent aspects of the issue   General answers are great  but I would also like to see details especially if they are only available on one platform

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One other thing  make sure the servers have the up to date daylight savings patch applied   We had a situation last year where our times were consistently out by one hour for a three-week period for North American users  even though we were using a UTC based system   It turns out in the end it was the servers  They just needed an up-to-date patch applied  Windows Server 2003

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If you happen to maintain database systems that are running with DST active  check carefully whether they need to be shut down during the transition in fall  Mandy DBS  or other systems as well  don t like passing the same point in  local  time twice  which is exactly what happens when you turn back the clock in fall  SAP has solved this with a  IMHO really neat  workaround - instead of turning back the clock  they just let the internal clock run at half the usual speed for two hours

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Keep your servers set to UTC  and make sure they all are configured for ntp or the equivalent   UTC avoids daylight savings time issues  and out-of-sync servers can cause unpredictable results that take a while to diagnose

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For the web  the rules aren t that complicated      Server-side  use UTC  Client-side  use Olson  Reason  UTC-offsets are not daylight savings-safe  e g  New York is EST  UTC - 5 Hours  part of the year  EDT  UTC - 4 Hours  rest of the year   For client-side time zone determination  you have two options   1  Have user set zone  Safer   Resources  Web-ready Olson tz HTML Dropdown and JSON  2  Auto-detect zone  Resource  jsTimezoneDetect      The rest is just UTC local conversion using your server-side datetime libraries  Good to go

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I have hit this on two types of systems     shift planning systems  e g  factory workers     and    gas depend management systems      23 and 25 hour long days are a pain to cope with  so are 8hr shifts that take 7hr or 9hr   The problem is you will find that each customers  or even department of the customer have different rules they have created  often without documenting  on what they do in these special cases   Some questions are best not asked of the customer   s until after they have paid for your    off the shelf    software   It is very rare to find a customer that thinks about this type of issue up front when buying software   I think in all cases you should record time in UTC and convert to from local time before storing the date time    However even know which take a given time is in can be hard with Daylight saving and time zones

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When it comes to applications that run on a server  including web sites and other back-end services  the time zone setting of the server should be ignored by the application   The common advice is to set the server s time zone to UTC   This is indeed a good best practice  but it s there as a band-aid for applications that do not follow other best practices   For example  a service might be writing to log files with local timestamps instead of UTC-based timestamps  thus creating ambiguities during the daylight saving time fall-back transition   Setting the server s time zone to UTC will fix that application   However the real fix would be for the application to log using UTC to begin with   Server-side code  including web sites  should never expect the local time zone of the server to be anything in particular   In some languages  the local time zone can easily creep in to application code   For example  the DateTime ToUniversalTime method in  NET will convert from the local time zone to UTC  and the DateTime Now property returns the current time in the local time zone    Also  the Date constructor in JavaScript uses the computer s local time zone   There are many other examples like this    It is important to practice defensive programming  avoiding any code that uses the computer s local time zone setting   Reserve using the local time zone for client-side code  such as desktop applications  mobile applications  and client-side JavaScript

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Crossing the boundary of  computer time  and  people time  is a nightmare  The main one being that there is no sort of standard for the rules governing timezones and daylight saving times  Countries are free to change their timezone and DST rules at any time  and they do   Some countries e g  Israel  Brazil  decide each year when to have their daylight saving times  so it is impossible to know in advance when  if  DST will be in effect  Others have fixed ish   rules as to when DST is in effect  Other countries do not use DST as all   Timezones do not have to be full hour differences from GMT  Nepal is  5 45  There are even timezones that are  13  That means that   SUN 23 00 in Howland Island  -12  MON 11 00 GMT  TUE 00 00 in Tonga   13    are all the same time  yet 3 different days   There is also no clear standard on the abbreviations for timezones  and how they change when in DST so you end up with things like this   AST Arab Standard Time     UTC 03 AST Arabian Standard Time  UTC 04 AST Arabic Standard Time   UTC 03   The best advice is to stay away from local times as much as possible and stick to UTC where you can  Only convert to local times at the last possible moment   When testing make sure you test countries in the Western and Eastern hemispheres  with both DST in progress and not and a country that does not use DST  6 in total

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Tom Scott s video about timezones on YouTube on the Computerphile channel has also a nice and entertaining description of the topic   Examples include     Samoa  an island in the Pacific Ocean  shifting its time zone forward by 24 hours to make trading easier with Australia and New Zealand   West Bank  where 2 populations of people are following different time zones  18th century changes from the Julian calendar to the Gregorian calendar  which happened in the 20th century in Russia

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In dealing with databases  in particular MySQL  but this applies to most databases   I found it hard to store UTC    Databases usually work with server datetime by default  that is  CURRENT TIMESTAMP   You may not be able to change the server timezone  Even if you are able to change the timezone  you may have third-party code that expects server timezone to be local    I found it easier to just store server datetime in the database  then let the database convert the stored datetime back to UTC  that is  UNIX TIMESTAMP    in the SQL statements  After that you can use the datetime as UTC in your code   If you have 100  control over the server and all code  it s probably better to change server timezone to UTC

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You need to know about the Olson tz database  which is available from ftp   elsie nci nih gov pub http   iana org time-zones    It is updated multiple times per year to deal with the often last-minute changes in when  and whether  to switch between winter and summer  standard and daylight saving  time in different countries around the world   In 2009  the last release was 2009s  in 2010  it was 2010n  in 2011  it was 2011n  at the end of May 2012  the release was 2012c   Note that there is a set of code to manage the data and the actual time zone data itself  in two separate archives  tzcode20xxy tar gz and tzdata20xxy tar gz    Both code and data are in the public domain   This is the source of time zone names such as America Los Angeles  and synonyms such as US Pacific    If you need to keep track of different zones  then you need the Olson database   As others have advised  you also want to store the data in a fixed format  mdash  UTC is normally the one chosen  mdash  along with a record of the time zone in which the data was generated   You may want to distinguish between the offset from UTC at the time and the time zone name  that can make a difference later   Also  knowing that it is currently 2010-03-28T23 47 00-07 00  US Pacific  may or may not help you with interpreting the value 2010-11-15T12 30  mdash  which is presumably specified in PST  Pacific Standard Time  rather than PDT  Pacific Daylight Saving Time    The standard C library interfaces are not dreadfully helpful with this sort of stuff     The Olson data has moved  in part because A D Olson will be retiring soon  and in part because there was a  now dismissed  law suit against the maintainers for copyright infringement   The time zone database is now managed under the auspices of IANA  the Internet Assigned Numbers Authority  and there s a link on the front page to  Time Zone Database    The discussion mailing list is now tz iana org  the announcement list is tz-announce iana org

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While I haven t tried it  an approach to time zone adjustments I would find compelling would be as follows    Store everything in UTC  Create a table TZOffsets with three columns  RegionClassId  StartDateTime  and OffsetMinutes  int  in minutes     In the table  store a list of dates and times when the local time changed  and by how much  The number of regions in the table and the number of dates would depend on what range of dates and areas of the world you need to support  Think of this as if it is  historical  date  even though the dates should include the future to some practical limit   When you need to compute the local time of any UTC time  just do this   SELECT DATEADD  m   SUM OffsetMinutes    inputdatetime  AS LocalDateTime FROM   TZOffsets WHERE  StartDateTime  lt    inputdatetime        AND RegionClassId    RegionClassId    You might want to cache this table in your app and use LINQ or some similar means to do the queries rather than hitting the database   This data can be distilled from the public domain tz database   Advantages and footnotes of this approach    No rules are baked into code  you can adjust the offsets for new regions or date ranges readily  You don t have to support every range of dates or regions  you can add them as needed  Regions don t have to correspond directly to geopolitical boundaries  and to avoid duplication of rows  for instance  most states in the US handle DST the same way   you can have broad RegionClass entries that link in another table to more traditional lists of states  countries  etc  For situations like the US where the start and end date of DST has changed over the past few years  this is pretty easy to deal with  Since the StartDateTime field can store a time as well  the 2 00 AM standard change-over time is handled easily  Not everywhere in the world uses a 1-hour DST  This handles those cases easily  The data table is cross-platform and could be a separate open-source project that could be used by developers who use nearly any database platform or programming language  This can be used for offsets that have nothing to do with time zones  For instance  the 1-second adjustments that happen from time to time to adjust for the Earth s rotation  historical adjustments to and within the Gregorian calendar  etc  Since this is in a database table  standard report queries  etc  can take advantage of the data without a trip through business logic code  This handles time zone offsets as well if you want it to  and can even account for special historical cases where a region is assigned to another time zone  All you need is an initial date that assigns a time zone offset to each region with a minimal start date  This would require creating at least one region for each time zone  but would allow you to ask interesting questions like   What is the difference in local time between Yuma  Arizona and Seattle  Washington on February 2  1989 at 5 00am    Just subtract one SUM   from the other     Now  the only disadvantage of this approach or any other is that conversions from local time to GMT are not perfect  since any DST change that has a negative offset to the clock repeats a given local time  No easy way to deal with that one  I m afraid  which is one reason storing local times is bad news in the first place

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For PHP   The DateTimeZone class in PHP   5 2 is already based on the Olson DB which others mention  so if you are doing timezone conversions in PHP and not in the DB  you are exempt of working with  the hard-to-understand  Olson files    However  PHP is not updated as frequently as the Olson DB  so just using PHPs time zone conversions may leave you with outdated DST information and influence the correctness of your data  While this is not expected to happen frequently  it may happen  and will happen if you have a large base of users worldwide    To cope with the above issue  use the timezonedb pecl package   Its function is to update PHP s timezone data  Install this package as frequently as it is updated   I m not sure if the updates to this package follow Olson updates exactly  but it seems to be updated at a frequency which is at least very close to the frequency of Olson updates

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Just one example to prove that handling time is the huge mess described  and that you can never be complacent  In several spots on this page leap-seconds have been ignored    Several years ago  the Android operating system used GPS satellites to get a UTC time reference  but ignored the fact that GPS satellites do not use leap-seconds  No one noticed until there was confusion on New Year s Eve  when the Apple phone users and Android phone users did their count-downs about 15 seconds apart    I think it has since been fixed  but you never know when these  minor details  will come back to haunt you

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Here is my experience -   Does not require any third-party library    On server-side  store times in UTC format so that all date time values in database are in a single standard regardless of location of users  servers  timezones or DST  On the UI layer or in emails sent out to user  you need to show times according to user  For that matter  you need to have user s timezone offset so that you can add this offset to your database s UTC value which will result in user s local time  You can either take user s timezone offset when they are signing up or you can auto-detect them in web and mobile platforms  For websites  JavaScript s function getTimezoneOffset   method is a standard since version 1 0 and compatible with all browsers   Ref  http   www w3schools com jsref jsref getTimezoneOffset asp

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Make clear architectural separation of concerns - to know exactly which tier interacts with users  and has to change date-time for from canonical representation  UTC   Non-UTC date-time is presentation  follows users local timezone   UTC time is model  remains unique for back-end and mid tiers    Also  decide what s your actual audience  what you don t have to serve and where do you draw the line  Don t touch exotic calendars unless you actually have important customers there and then consider separate user-facing server s  just for that region   If you can acquire and maintain user s location  use location for systematic date-time conversion  say  NET culture or a SQL table  but provide a way for end-user to choose overrides if date-time is critical for your users   If there are historical audit obligations involved  like telling exactly when Jo in AZ paid a bill 2 yrs ago in September  then keep both UTC and local time for the record  your conversion tables will change in a course of time    Define the time referential time zone for data that comes in bulk - like files  web services etc  Say East Coast company has data center in CA - you need to ask and know what they use as a standard instead of assuming one or the other   Don t trust time-zone offsets embedded in textual representation of the date-time and don t accept to parse and follow them  Instead always request that time zone and or reference zone have to be explicitly defined  You can easily receive time with PST offset but the time is actually EST since that s the client s reference time and records were just exported at a server which is in PST

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For those struggling with this on  NET  see if using DateTimeOffset and or TimeZoneInfo are worth your while   If you want to use IANA Olson time zones  or find the built in types are insufficient for your needs  check out Noda Time  which offers a much smarter date and time API for  NET

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I recently had a problem in a web application where on an Ajax post-back the datetime coming back to my server-side code was different from the datetime served out   It most likely had to do with my JavaScript code on the client that built up the date for posting back to the client as string  because JavaScript was adjusting for time zone and daylight savings  and in some browsers the calculation for when to apply daylight savings seemed to be different than in others   In the end I opted to remove date and time calculations on the client entirely  and posted back to my server on an integer key which then got translated to date time on the server  to allow for consistent transformations   My learning from this  Do not use JavaScript date and time calculations in web applications unless you ABSOLUTELY have to

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Summary of answers and other data   please add yours   Do    Whenever you are referring to an exact moment in time  persist the time according to a unified standard that is not affected by daylight savings   GMT and UTC are equivalent with this regard  but it is preferred to use the term UTC  Notice that UTC is also known as Zulu or Z time   If instead you choose to persist a time using a local time value  include the local time offset for this particular time from UTC  this offset may change throughout the year   such that the timestamp can later be interpreted unambiguously   In some cases  you may need to store both the UTC time and the equivalent local time   Often this is done with two separate fields  but some platforms support a datetimeoffset type that can store both in a single field  When storing timestamps as a numeric value  use Unix time - which is the number of whole seconds since 1970-01-01T00 00 00Z  excluding leap seconds    If you require higher precision  use milliseconds instead   This value should always be based on UTC  without any time zone adjustment  If you might later need to modify the timestamp  include the original time zone ID so you can determine if the offset may have changed from the original value recorded  When scheduling future events  usually local time is preferred instead of UTC  as it is common for the offset to change   See answer  and blog post  When storing whole dates  such as birthdays and anniversaries  do not convert to UTC or any other time zone    When possible  store in a date-only data type that does not include a time of day  If such a type is not available  be sure to always ignore the time-of-day when interpreting the value   If you cannot be assured that the time-of-day will be ignored  choose 12 00 Noon  rather than 00 00 Midnight as a more safe representative time on that day   Remember that time zone offsets are not always an integer number of hours  for example  Indian Standard Time is UTC 05 30  and Nepal uses UTC 05 45   If using Java  use java time for Java 8 and later     Much of that java time functionality is back-ported to Java 6  amp  7 in the ThreeTen-Backport library   Further adapted for early Android   lt  26  in the ThreeTenABP library   These projects officially supplant the venerable Joda-Time  now in maintenance-mode  Joda-Time  ThreeTen-Backport  ThreeTen-Extra  java time classes  and JSR 310 are led by the same man  Stephen Colebourne   If using  NET  consider using Noda Time  If using  NET without Noda Time  consider that DateTimeOffset is often a better choice than DateTime  If using Perl  use DateTime  If using Python  use pytz or dateutil  If using JavaScript  use moment js with the moment-timezone extension  If using PHP   5 2  use the native time zones conversions provided by DateTime  and DateTimeZone classes  Be careful when using DateTimeZone  listAbbreviations   - see answer  To keep PHP with up to date Olson data  install periodically the timezonedb PECL package  see answer  If using C    be sure to use a library that uses the properly implements the IANA timezone database   These include cctz  ICU  and Howard Hinnant s  tz  library    Do not use Boost for time zone conversions   While its API claims to support standard IANA  aka  zoneinfo   identifiers  it crudely maps them to POSIX-style data  without considering the rich history of changes each zone may have had    Also  the file has fallen out of maintenance    If using Rust  use chrono  Most business rules use civil time  rather than UTC or GMT   Therefore  plan to convert UTC timestamps to a local time zone before applying application logic  Remember that time zones and offsets are not fixed and may change  For instance  historically US and UK used the same dates to  spring forward  and  fall back   However  in 2007 the US changed the dates that the clocks get changed on  This now means that for 48 weeks of the year the difference between London time and New York time is 5 hours and for 4 weeks  3 in the spring  1 in the autumn  it is 4 hours  Be aware of items like this in any calculations that involve multiple zones  Consider the type of time  actual event time  broadcast time  relative time  historical time  recurring time  what elements  timestamp  time zone offset and time zone name  you need to store for correct retrieval - see  Types of Time  in this answer  Keep your OS  database and application tzdata files in sync  between themselves and the rest of the world  On servers  set hardware clocks and OS clocks to UTC rather than a local time zone  Regardless of the previous bullet point  server-side code  including web sites  should never expect the local time zone of the server to be anything in particular   see answer  Prefer working with time zones on a case-by-case basis in your application code  rather than globally through config file settings or defaults  Use NTP services on all servers  If using FAT32  remember that timestamps are stored in local time  not UTC  When dealing with recurring events  weekly TV show  for example   remember that the time changes with DST and will be different across time zones  Always query date-time values as lower-bound inclusive  upper-bound exclusive   gt     lt      Don t    Do not confuse a  time zone   such as America New York with a  time zone offset   such as -05 00   They are two different things   See the timezone tag wiki  Do not use JavaScript s Date object to perform date and time calculations in older web browsers  as ECMAScript 5 1 and lower has a design flaw that may use daylight saving time incorrectly    This was fixed in ECMAScript 6   2015   Never trust the client s clock  It may very well be incorrect   Don t tell people to  always use UTC everywhere    This widespread advice is shortsighted of several valid scenarios that are described earlier in this document   Instead  use the appropriate time reference for the data you are working with    Timestamping can use UTC  but future time scheduling and date-only values should not     Testing    When testing  make sure you test countries in the Western  Eastern  Northern and Southern hemispheres  in fact in each quarter of the globe  so 4 regions   with both DST in progress and not  gives 8   and a country that does not use DST  another 4 to cover all regions  making 12 in total    Test transition of DST  i e  when you are currently in summer time  select a time value from winter  Test boundary cases  such as a timezone that is UTC 12  with DST  making the local time UTC 13 in summer and even places that are UTC 13 in winter Test all third-party libraries and applications and make sure they handle time zone data correctly  Test half-hour time zones  at least    Reference    The detailed timezone tag wiki page on Stack Overflow Olson database  aka Tz database IETF draft procedures for maintaining the Olson database Sources for Time Zone and DST ISO format  ISO 8601  Mapping between Olson database and Windows Time Zone Ids  from the Unicode Consortium Time Zone page on Wikipedia StackOverflow questions tagged dst StackOverflow questions tagged timezone Dealing with DST - Microsoft DateTime best practices Network Time Protocol on Wikipedia   Other    Lobby your representative to end the abomination that is DST  We can always hope    Lobby for Earth Standard Time

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Never rely only on constructors like    new DateTime int year  int month  int day  int hour  int minute  TimeZone timezone    They can throw exceptions when a certain date time does not exist due to DST  Instead  build your own methods for creating such dates  In them  catch any exceptions that occur due to DST  and adjust the time is needed with the transition offset  DST may occur on different dates and at different hours  even at midnight for Brazil  according to the timezone

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This is an important and surprisingly tough issue  The truth is that there is no completely satisfying standard for persisting time  For example  the SQL standard and the ISO format  ISO 8601  are clearly not enough   From the conceptual point of view  one usually deals with two types of time-date data  and it s convenient to distinguish them  the above standards do not     physical time  and  civil time     A  physical  instant of time is a point in the continuous universal timeline that physics deal with  ignoring relativity  of course   This concept can be adequately coded-persisted in UTC  for example  if you can ignore leap seconds     A  civil  time is a datetime specification that follows civil norms  a point of time here is fully specified by a set of datetime fields  Y M D H MM S FS  plus a TZ  timezone specification   also a  calendar   actually  but lets assume we restrict the discussion to Gregorian calendar   A timezone and a calendar jointly allow  in principle  to map from one representation to another  But civil and physical time instants are fundamentally different types of magnitudes  and they should be kept conceptually separated and treated differently   an analogy  arrays of bytes and character strings    The issue is confusing because we speak of these types events interchangeably  and because the civil times are subject to political changes  The problem  and the need to distinguish these concepts  becomes more evident for events in the future  Example  taken from my discussion here   John records in his calendar a reminder for some event at datetime 2019-Jul-27  10 30 00  TZ Chile Santiago    which has offset GMT-4  hence it corresponds to  UTC 2019-Jul-27 14 30 00   But some day in the future  the country decides to change the TZ offset to GMT-5   Now  when the day comes    should that reminder trigger at  A  2019-Jul-27 10 30 00  Chile Santiago    UTC time  2019-Jul-27 15 30 00    or  B  2019-Jul-27  9 30 00  Chile Santiago    UTC time  2019-Jul-27 14 30 00    There is no correct answer  unless one knows what John conceptually meant when he told the calendar  Please ring me at 2019-Jul-27  10 30 00 TZ Chile Santiago    Did he mean a  civil date-time    when the clocks in my city tell 10 30    In that case  A  is the correct answer   Or did he mean a   physical instant of time   a point in the continuus line of time of our universe  say   when the next solar eclipse happens   In that case  answer B  is the correct one   A few Date Time APIs get this distinction right  among them  Jodatime  which is the foundation of the next  third   Java DateTime API  JSR  310

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PHP s DateTimeZone  listAbbreviations   output  This PHP method returns an associative array containing some  major  timezones  like CEST   which on their own contain more specific  geographic  timezones  like Europe Amsterdam    If you re using these timezones and their offset DST information  it s extremely important to realize the following   It seems like all different offset DST configurations  including historical configurations  of each timezone are included   For example  Europe Amsterdam can be found six times in the output of this function  Two occurrences  offset 1172 4772  are for the Amsterdam time used until 1937  two  1200 4800  are for the time that was used between 1937 and 1940  and two  3600 4800  are for the time used since 1940   Therefore  you cannot rely on the offset DST information returned by this function as being currently correct in use   If you want to know the current offset DST of a certain timezone  you ll have to do something like this    lt  php  now   new DateTime null  new DateTimeZone  Europe Amsterdam     echo  now- gt getOffset      gt

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Are you using the  NET framework  If so  let me introduce you to the DateTimeOffset type  added with  NET 3 5   This structure holds both a DateTime and an Offset  TimeSpan   which specifies the difference between the DateTimeOffset instance s date and time and Coordinated Universal Time  UTC     The DateTimeOffset Now static method will return a DateTimeOffset instance consisting of the current  local  time  and the local offset  as defined in the operating system s regional info   The DateTimeOffset UtcNow static method will return a DateTimeOffset instance consisting of the current time in UTC  as if you were in Greenwich     Other helpful types are the TimeZone and TimeZoneInfo classes

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Just wanted to point out two things that seem inaccurate or at least confusing      Always persist time according to a unified standard that is not   affected by daylight savings  GMT and UTC have been mentioned by   different people  though UTC seems to be mentioned most often    For  almost  all practical computing purposes  UTC is  in fact  GMT  Unless you see a timestamps with a fractional second  you re dealing with GMT which makes this distinction redundant      Include the local time offset as is  including DST offset  when   storing timestamps    A timestamp is always represented in GMT and thus has no offset

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If your design can accommodate it  avoid local time conversion all together    I know to some this might sound insane but think about UX  users process near  relative dates  today  yesterday  next Monday  faster than absolute dates  2010 09 17  Friday Sept 17  on glance  And when you think about it more  the accuracy of timezones  and DST  is more important the closer the date is to now    so if you can express dates datetimes in a relative format for   - 1 or 2 weeks  the rest of the dates can be UTC and it wont matter too much to 95  of users   This way you can store all dates in UTC and do the relative comparisons in UTC and simply show the user UTC dates outside of your Relative Date Threshold   This can also apply to user input too  but generally in a more limited fashion   Selecting from a drop down that only has   Yesterday  Today  Tomorrow  Next Monday  Next Thursday   is so much simpler and easier for the user than a date picker  Date pickers are some of the most pain inducing components of form filling  Of course this will not work for all cases but you can see that it only takes a little clever design to make it very powerful

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Be careful when dealing with timestamps stored in the FAT32 filesystem - it is always persisted in local time coordinates  which include DST - see msdn article    Got burned on that one

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I m not sure what I can add to the answers above  but here are a few points from me   Types of times  There are four different times you should consider    Event time  eg  the time when an international sporting event happens  or a coronation death etc   This is dependent on the timezone of the event and not of the viewer  Television time  eg  a particular TV show is broadcast at 9pm local time all around the world   Important when thinking about publishing the results  of say American Idol  on your website Relative time  eg  This question has an open bounty closing in 21 hours   This is easy to display Recurring time  eg  A TV show is on every Monday at 9pm  even when DST changes    There is also Historic alternate time   These are annoying because they may not map back to standard time   Eg  Julian dates  dates according to a Lunar calendar on Saturn  The Klingon calendar   Storing start end timestamps in UTC works well   For 1  you need an event timezone name   offset stored along with the event   For 2  you need a local time identifier stored with each region and a local timezone name   offset stored for every viewer  it s possible to derive this from the IP if you re in a crunch    For 3  store in UTC seconds and no need for timezones   4 is a special case of 1 or 2 depending on whether it s a global or a local event  but you also need to store a created at timestamp so you can tell if a timezone definition changed before or after this event was created   This is necessary if you need to show historic data   Storing times   Always store time in UTC Convert to local time on display  local being defined by the user looking at the data  When storing a timezone  you need the name  timestamp and the offset   This is required because governments sometimes change the meanings of their timezones  eg  the US govt changed DST dates   and your application needs to handle things gracefully    eg  The exact timestamp when episodes of LOST showed both before and after DST rules changed    Offsets and names  An example of the above would be      The soccer world cup finals game   happened in South Africa  UTC 2--SAST    on July 11  2010 at 19 00 UTC    With this information  we can historically determine the exact time when the 2010 WCS finals took place even if the South African timezone definition changes  and be able to display that to viewers in their local timezone at the time when they query the database   System Time  You also need to keep your OS  database and application tzdata files in sync  both with each other  and with the rest of the world  and test extensively when you upgrade   It s not unheard of that a third party app that you depend on did not handle a TZ change correctly   Make sure hardware clocks are set to UTC  and if you re running servers around the world  make sure their OSes are configured to use UTC as well   This becomes apparent when you need to copy hourly rotated apache log files from servers in multiple timezones   Sorting them by filename only works if all files are named with the same timezone   It also means that you don t have to do date math in your head when you ssh from one box to another and need to compare timestamps   Also  run ntpd on all boxes   Clients  Never trust the timestamp you get from a client machine as valid   For example  the Date  HTTP headers  or a javascript Date getTime   call   These are fine when used as opaque identifiers  or when doing date math during a single session on the same client  but don t try to cross-reference these values with something you have on the server   Your clients don t run NTP  and may not necessarily have a working battery for their BIOS clock   Trivia  Finally  governments will sometimes do very weird things      Standard time in the Netherlands was   exactly 19 minutes and 32 13 seconds   ahead of UTC by law from 1909-05-01   through 1937-06-30  This time zone   cannot be represented exactly using   the HH MM format    Ok  I think I m done

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In general  include the local time offset  including DST offset  in stored timestamps  UTC alone is not enough if you later want to display the timestamp in its original timezone  and DST setting    Keep in mind that the offset is not always an integer number of hours  e g  Indian Standard Time is UTC 05 30    For example  suitable formats are a tuple  unix time  offset in minutes  or ISO 8601

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Business rules should always work on civil time  unless there s legislation that says otherwise   Be aware that civil time is a mess  but it s what people use so it s what is important   Internally  keep timestamps in something like civil-time-seconds-from-epoch  The epoch doesn t matter particularly  I favour the Unix epoch  but it does make things easier than the alternative  Pretend that leap-seconds don t exist unless you re doing something that really needs them  e g   satellite tracking   The mapping between timestamps and displayed time is the only point where DST rules should be applied  the rules change frequently  on a global level  several times a year  blame politicians  so you should make sure that you do not hard-code the mapping  Olson s TZ database is invaluable

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Never  ever store local time without its UTC offset  or a reference to the time zone    examples of how not to do it include FAT32 and struct tm in C    Understand that a time zone is a combination of   a set of UTC offsets  e g   0100 in winter   0200 in summer  rules for when the switchover happens  which may change over time  for example  in the 1990s the EU harmonized the switchover as being on the last Sundays in March and October  at 02 00 standard time 03 00 DST  previously this differed between member states           Some implementations of struct tm do store the UTC offset  but this has not made it into the standard

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Actually  kernel32 dll does not export SystemTimeToTzSpecificLocation   It does however export the following two   SystemTimeToTzSpecificLocalTime and TzSpecificLocalTimeToSystemTime

[datetime] Daylight saving time and time zone best practices

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