How to check if DST Daylight Saving Time is in effect and if so the offset

Question

This is a bit of my JS code for which this is needed   var secDiff   Math abs Math round  utc date-this premiere date  1000    this years   this calculateUnit secDiff  86400 365    this days   this calculateUnit secDiff- this years  86400 365   86400   this hours   this calculateUnit  secDiff- this years  86400 365  - this days 86400   3600   this minutes   this calculateUnit  secDiff- this years  86400 365  - this days 86400 - this hours 3600   60   this seconds   this calculateUnit  secDiff- this years  86400 365  - this days 86400 - this hours 3600 - this minutes 60   1     I want to get the datetime in  ago   but if the DST is in use then the dates are off by 1 hour  I don t know how to check if the DST is in effect or not   How can I know when the daylight saving starts and ends

User · Answer

The moment js library provides an  isDst   method on its time objects      moment isDST checks if the current moment is in daylight saving time    moment  2011  2  12   isDST       false  March 12 2011 is not DST moment  2011  2  14   isDST       true  March 14 2011 is DST

User · Answer

Use Moment js  https   momentjs com     moment   isDST    will give you if Day light savings is observed   Also it has helper function to calculate relative time for you  You don t need to do manual calculations e g moment  20200105    YYYYMMDD   fromNow

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ES6 Style  Math min     0  6  map v   gt  new Date 95  v  1  getTimezoneOffset     -1

User · Answer

This answer is quite similar to the accepted answer  but doesn t override the Date prototype  and only uses one function call to check if Daylight Savings Time is in effect  rather than two     The idea is that  since no country observes DST that lasts for 7 months 1   in an area that observes DST the offset from UTC time in January will be different to the one in July    While Daylight Savings Time moves clocks forwards  JavaScript always returns a greater value during Standard Time  Therefore  getting the minimum offset between January and July will get the timezone offset during DST   We then check if the dates timezone is equal to that minimum value  If it is  then we are in DST  otherwise we are not   The following function uses this algorithm  It takes a date object  d  and returns true if daylight savings time is in effect for that date  and false if it is not   function isDST d        let jan   new Date d getFullYear    0  1  getTimezoneOffset        let jul   new Date d getFullYear    6  1  getTimezoneOffset        return Math max jan  jul     d getTimezoneOffset

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Future-Proof Solution That Works In All Time Zones   Let x be the expected number of milliseconds into the year of interest without factoring in daylight savings  Let y be the number of milliseconds since the Epoch from the start of the year of the date of interest  Let z be the number of milliseconds since the Epoch of the full date and time of interest Let t be the subtraction of both x and y from z  z - y - x  This yields the offset due to DST   If t is zero  then DST is not in effect  If t is not zero  then DST is in effect     x000D   x000D   function    use strict   x000D  function dstOffsetAtDate dateInput    x000D      var fullYear   dateInput getFullYear   0  x000D       Leap Years are any year that can be exactly divided by 4  2012  2016  etc  x000D         except if it can be exactly divided by 100  then it isn t  2100 2200 etc  x000D         except if it can be exactly divided by 400  then it is  2000  2400   x000D       https   www mathsisfun com leap-years html   x000D      var isLeapYear     fullYear  amp  3     fullYear 100  amp  3       0   1   0  x000D       fullYear  amp  3     fullYear   4   but faster x000D        Alternative var isLeapYear  new Date currentYear 1 29 12   getDate     29 1 0 x000D      var fullMonth   dateInput getMonth   0  x000D      return   x000D             1  We know what the time since the Epoch really is x000D            dateInput     same as the dateInput getTime   method x000D             2  We know what the time since the Epoch at the start of the year is x000D          -   new Date fullYear  0  0      day defaults to 1 if not explicitly zeroed x000D             3  Now  subtract what we would expect the time to be if daylight savings x000D                  did not exist  This yields the time-offset due to daylight savings  x000D          -    x000D                 x000D                     Calculate the day of the year in the Gregorian calendar x000D                     The code below works based upon the facts of signed right shifts x000D                             x   gt  gt  n  shifts n and fills in the n highest bits with 0s  x000D                             -x   gt  gt  n  shifts n and fills in the n highest bits with 1s x000D                      This assumes that x is a positive integer  x000D                   31  amp    -fullMonth   gt  gt  4        January     -11  gt  gt 4   -1 x000D                    28   isLeapYear   amp    1-fullMonth   gt  gt  4        February x000D                   31  amp    2-fullMonth   gt  gt  4        March x000D                   30  amp    3-fullMonth   gt  gt  4        April x000D                   31  amp    4-fullMonth   gt  gt  4        May x000D                   30  amp    5-fullMonth   gt  gt  4        June x000D                   31  amp    6-fullMonth   gt  gt  4        July x000D                   31  amp    7-fullMonth   gt  gt  4        August x000D                   30  amp    8-fullMonth   gt  gt  4        September x000D                   31  amp    9-fullMonth   gt  gt  4        October x000D                   30  amp    10-fullMonth   gt  gt  4        November x000D                     There are no months past December  the year rolls into the next  x000D                     Thus  fullMonth is 0-based  so it will never be 12 in Javascript x000D                   x000D                   dateInput getDate   0     get day of the month x000D       x000D                amp 0xffff    24   60    24 hours in a day  60 minutes in an hour x000D                 dateInput getHours   amp 0xff    60    60 minutes in an hour x000D                 dateInput getMinutes   amp 0xff  x000D            0    60   1000    60 seconds in a minute   1000 milliseconds in a second x000D          -  dateInput getSeconds   amp 0xff    1000    1000 milliseconds in a second x000D          - dateInput getMilliseconds   x000D         x000D    x000D   x000D     Demonstration  x000D  var date   new Date 2100  0  1  x000D  for  var i 0  i lt 12  i i 1 0  date setMonth date getMonth   1 0   x000D      console log date getMonth      t  dstOffsetAtDate date  60 60 1000  h t  date   x000D  date   new Date 1900  0  1   x000D  for  var i 0  i lt 12  i i 1 0  date setMonth date getMonth   1 0   x000D      console log date getMonth      t  dstOffsetAtDate date  60 60 1000  h t  date   x000D   x000D     Performance Benchmark  x000D  console time  Speed of processing 16384 dates    x000D  for  var i 0 month date getMonth   0  i lt 16384  i i 1 0  x000D      date setMonth month month 1  dstOffsetAtDate date  0  0   x000D  console timeEnd  Speed of processing 16384 dates    x000D        x000D   x000D   x000D    I believe that the above code snippet is superior to all other answers posted here for many reasons    This answer works in all time zones  even Antarctica Casey  Daylight savings is very much subject to change  It might be that 20 years from now  some country might have 3 DST periods instead of the normal 2  This code handles that case by returning the DST offset in milliseconds  not just whether DST is in effect or not in effect  The size of the months of the year and the way that Leap Years work fits perfectly into keeping our time on track with the sun  Heck  it works so perfectly that all we ever do is just adjust mere seconds here and there  Our current system of leap years has been in effect since February 24th  1582  and will likely stay in effect for the foreseeable future  This code works in timezones that do not use DST  This code works in historic times before when DST was implemented  such as the 1900s   This code is maximally integer-optimized and should give you no problem if called in a tight loop  After running the code snippet above  scroll down to the bottom of the output to see the performance benchmark  My computer is able to process 16384 dates in  97ms on Chrome    However  if you are not preparing for over 2 DST periods  then the below code can be used to determine whether DST is in effect as a boolean   function isDaylightSavingsInEffect dateInput           To satisfy the original question     return dstOffsetAtDate dateInput      0

User · Answer

Create two dates  one in June  one in January  Compare their getTimezoneOffset   values   if January offset  gt  June offset  client is in northern hemisphere if January offset  lt  June offset  client is in southern hemisphere if no difference  client timezone does not observe DST  Now check getTimezoneOffset   of the current date   if equal to June  northern hemisphere  then current time zone is DST   1 hour  if equal to January  southern hemisphere  then current time zone is DST   1 hour

User · Answer

The getTimezoneOffset   method in JavaScript  in a browser  returns the number of minutes offset from the 00 00 time zone   For example  America New York time zone in Daylight Savings  DST  returns the number 300   300 minutes is 5 hours difference from zero   300 minutes divided by 60 minutes is 5 hours   Every time zone is compared to the zero time zone   00 00   Etc GMT   Greenwich time   MDN Web Docs  The next thing that you must know  is that the offset has the opposite sign of the actual time zone     Information about time zones is maintained by the Internet Assigned Numbers Authority  iana   iana time zones  A nicely formatted table of Time Zones is supplied by joda org  joda-time Time Zones   00 00 or Etc GMT is Greenwich time  All time zones are offset from  00 00    Etc GMT    Greenwich time  Daylight Savings Time is always an earlier time than the  regular  time in the summer   You set your clocks back in the fall season     Fall Back  slogan to remember what to do   So  America New York time in Daylight Savings  winter  is one hour before the regular time   So  for example  what was normally 5 p m  in the afternoon in New York city in the summer  is now 4 p m  America New York time in Daylight Savings   The name  America New York  time is a  Long Format  time zone name   The east coast of the U S typically calls their time zone Eastern Standard Time  EST   If you want to compare today s time zone offset to the time zone offset of some other date  you need to know that mathematical sign    -  Positive   Negative   of the time zone offset is the opposite of the time zone   Look at the time zone table at joda org and find the time zone for  America New York   It will have a negative sign in front of the Standard Offset   The earth rotates counter-clockwise on it s axis   A person watch the sunrise in Greenwich sees the sunrise 5 hours before someone in New York City will see the sunrise   And someone on the West Coast of the U S  will see the sunrise after someone on the East Coast of the U S  sees the sunrise   There s a reason why you need to know all of this   So that you ll be able to logically determine whether some JavaScript code is getting the DST status correctly or not  without needing to test every time zone at different times of the year   Imagine that it s November in New York City  and the clocks have been set back an hour   In the summer in New York City  the offset is 240 minutes or 4 hours   You can test this by creating a date that is in July and then getting the offset   var July Date   new Date 2017  6  1   var july Timezone OffSet   July Date getTimezoneOffset     console log  july Timezone OffSet      july Timezone OffSet    What will print to the browser s developer tools console log   Answer is  240  So  now you can create a date in January and see what your browser returns for a time zone offset for the winter season   var Jan Date   new Date 2017  0  1    Month is zero indexed - Jan is zero var jan Timezone OffSet   Jan Date getTimezoneOffset     console log  jan Timezone OffSet      jan Timezone OffSet    Answer is  300  Obviously 300 is bigger than 240   So  what does this mean   Should you write code that tests for the winter offset being bigger than the summer offset   Or the summer offset less than the winter offset   If there is a difference between the summer and winter time zone offsets  then you can assume that DST is being used for this time zone   But that doesn t tell you if today is using DST for the browsers time zone   So  you ll need to get the time zone offset for today   var today   new Date    var todaysTimeZone   today getTimezoneOffset     console log  todaysTimeZone       todaysTimeZone    Answer is    - Depends on the time of year  If today s time zone offset and the summer time zone offset is the same  AND the summer and winter time zone offsets are different  then by logical deduction  today must be NOT be in DST   Can you omit comparing the summer and winter time zone offsets   To know if DST is used for this time zone  and just compare today s time zone offset to the summer TZ offset  and always get the correct answer   today s TZ Offset     Summer TZ Offset   Well  is today in the winter or summer   If you knew that then you could apply the following logic   if   it is winter  amp  amp    todays TZ Offset     summer TZ Offset      var are We In DST   true      But the problem is  that you don t know if today s date is in winter or summer   Every time zone can have it s own rules for when DST starts and stops   You d need to keep track of every time zone s rules for every time zone in the world   So  if there is a better and easier way then you might as well do it the better and easier way   What we are left with  is that you need to know if this time zone uses DST  and then compare today s time zone offset with the summer time zone offset   That will always give you a reliable answer   The final logic is   if   DST Is Used In This Time Zone  amp  amp    todays TZ Offset     summer TZ Offset      var are We In DST   true      Function to determine if the time zone in the browser uses DST   function is DST Used In This TimeZone       var Jan Date  jan Timezone OffSet  July Date  july Timezone OffSet        offsetsNotEqual  thisYear  today     today   new Date     Create a date object that is now   thisYear   today getFullYear     Get the year as a number    Jan Date   new Date thisYear  0  1    Month is zero indexed - Jan is zero   jan Timezone OffSet   Jan Date getTimezoneOffset       console log  jan Timezone OffSet      jan Timezone OffSet     July Date   new Date thisYear  6  1     july Timezone OffSet   July Date getTimezoneOffset       console log  july Timezone OffSet      july Timezone OffSet     offsetsNotEqual   july Timezone OffSet     jan Timezone OffSet   True if not equal    console log  offsetsNotEqual      offsetsNotEqual      return offsetsNotEqual   If the offsets are not equal for summer and          winter then the only possible reason is that DST is used for          this time zone

User · Answer

Based on Matt Johanson s comment on the solution provided by Sheldon Griffin I created the following code       Date prototype stdTimezoneOffset   function             var fy this getFullYear            if   Date prototype stdTimezoneOffset cache hasOwnProperty fy                  var maxOffset   new Date fy  0  1  getTimezoneOffset                var monthsTestOrder  6 7 5 8 4 9 3 10 2 11 1                for var mi 0 mi lt 12 mi                      var offset new Date fy  monthsTestOrder mi   1  getTimezoneOffset                    if  offset  maxOffset                         maxOffset Math max maxOffset offset                       break                                              Date prototype stdTimezoneOffset cache fy  maxOffset                    return Date prototype stdTimezoneOffset cache fy               Date prototype stdTimezoneOffset cache          Date prototype isDST   function             return this getTimezoneOffset    lt  this stdTimezoneOffset              It tries to get the best of all worlds taking into account all the comments and previously suggested answers and specifically it   1  Caches the result for per year stdTimezoneOffset so that you don t need to recalculate it when testing multiple dates in the same year   2  It does not assume that DST  if it exists at all  is necessarily in July     and will work even if it will at some point and some place be any month     However Performance-wise it will work faster if indeed July  or near by months  are indeed DST   3  Worse case it will compare the getTimezoneOffset of the first of each month   and do that Once per tested year    The assumption it does still makes is that the if there is DST period is larger then a single month     If someone wants to remove that assumption he can change loop into something more like whats in the solutin provided by Aaron Cole - but I would still jump half a year ahead and break out of the loop when two different offsets are found

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I ve found that using the Moment js library with some of the concepts described here  comparing Jan to June  works very well   This simple function will return whether the timezone that the user is in observes Daylight Saving Time   function HasDST         return moment  2017  1  1   isDST      moment  2017  6  1   isDST        A simple way to check that this works  on Windows  is to change your timezone to a non DST zone  for example Arizona will return false  whereas EST or PST will return true

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This code uses the fact that getTimezoneOffset returns a greater value during Standard Time versus Daylight Saving Time  DST   Thus it determines the expected output during Standard Time  and it compares whether the output of the given date the same  Standard  or less  DST     Note that getTimezoneOffset returns positive numbers of minutes for zones west of UTC  which are usually stated as negative hours  since they re  behind  UTC   For example  Los Angeles is UTC   8h Standard  UTC-7h DST  getTimezoneOffset returns 480  positive 480 minutes  in December  winter  Standard Time   rather than -480  It returns negative numbers for the Eastern Hemisphere  such -600 for Sydney in winter  despite this being  ahead   UTC 10h     Date prototype stdTimezoneOffset   function          var jan   new Date this getFullYear    0  1       var jul   new Date this getFullYear    6  1       return Math max jan getTimezoneOffset    jul getTimezoneOffset        Date prototype isDstObserved   function          return this getTimezoneOffset    lt  this stdTimezoneOffset       var today   new Date    if  today isDstObserved           alert   Daylight saving time

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Is there an issue using the Date toString   indexOf  Daylight Time    gt  -1       new Date       Sat Jan 01 100050 00 00 00 GMT-0500  Eastern Standard Time         new Date          Sun May 01 100033 00 00 00 GMT-0400  Eastern Daylight Time    This seems compatible with all browsers

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I recently needed to create a date string with UTC and DST  and based on Sheldon s answer I put this together    x000D   x000D  Date prototype getTimezone   function showDST    x000D      var jan   new Date this getFullYear    0  1   x000D      var jul   new Date this getFullYear    6  1   x000D   x000D      var utcOffset   new Date   getTimezoneOffset     60   -1  x000D      var dstOffset    jan getTimezoneOffset   - jul getTimezoneOffset      60  x000D   x000D      var utc    UTC    utcOffset getSign      utcOffset   100  preFixed 1000   x000D      var dst    DST    dstOffset getSign      dstOffset   100  preFixed 1000   x000D   x000D      if  showDST    x000D          return utc          dst        x000D        x000D   x000D      return utc  x000D    x000D  Number prototype preFixed   function  preCeiling    x000D      var num   parseInt this  10   x000D      if  preCeiling  amp  amp  num  lt  preCeiling    x000D          num   Math abs num   x000D          var numLength     num toString   length  x000D          var preCeilingLength   preCeiling toString   length  x000D          var preOffset     preCeilingLength - numLength  x000D          for  var i   0  i  lt  preOffset  i      x000D              num    0    num  x000D            x000D        x000D      return num  x000D    x000D  Number prototype getSign   function      x000D      var num    parseInt this  10   x000D      var sign        x000D      if  num  lt  0    x000D          sign    -   x000D        x000D      return sign  x000D    x000D   x000D  document body innerHTML    new Date   getTimezone       lt br gt    x000D  document body innerHTML    new Date   getTimezone true   x000D   lt p gt Output for Turkey  UTC 0200  and currently in DST   amp nbsp  UTC 0300  DST 0100  lt  p gt  x000D   lt hr gt  x000D   x000D   x000D

User · Answer

I was faced with this same problem today but since our daylight saving starts and stops at differing times from the USA  at least from my understanding   I used a slightly different route    var arr       for  var i   0  i  lt  365  i       var d   new Date     d setDate i    newoffset   d getTimezoneOffset     arr push newoffset     DST   Math min apply null  arr   nonDST   Math max apply null  arr     Then you simply compare the current timezone offset with DST and nonDST to see which one matches

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Your re close but a little off  You never need to calculate your own time as it is a result of your own clock  It can detect if you are using daylight saving time in your location but not for a remote location produced by the offset   newDateWithOffset   new Date utc    3600000  offset       This will still be wrong and off an hour if they are in DST  You need for a remote time account if they are currently inside their DST or not and adjust accordingly  try calculating this and change your clock to - lets say 2 1 2015 and reset the clock back an hour as if outside DST  Then calculate for an offset for a place that should still be 2 hours behind  It will show an hour ahead of the two hour window  You would still need to account for the hour and adjust  I did it for NY and Denver and always go the incorrect  hour ahead  in Denver

[javascript] How to check if DST (Daylight Saving Time) is in effect, and if so, the offset?

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