Why does Date parse give incorrect results

Question

Case One   new Date Date parse  Jul 8  2005       Output   Fri Jul 08 2005 00 00 00 GMT-0700  PST   Case Two   new Date Date parse  2005-07-08       Output   Thu Jul 07 2005 17 00 00 GMT-0700  PST     Why is the second parse incorrect

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There is some method to the madness  As a general rule  if a browser can interpret a date as an ISO-8601  it will   2005-07-08  falls into this camp  and so it is parsed as UTC   Jul 8  2005  cannot  and so it is parsed in the local time   See JavaScript and Dates  What a Mess  for more

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Here is a short  flexible snippet to convert a datetime-string in a cross-browser-safe fashion as nicel detailed by  drankin2112   var inputTimestamp    2014-04-29 13 00 15     example  var partsTimestamp   inputTimestamp split       -  g   if partsTimestamp length  lt  6        partsTimestamp   partsTimestamp concat   00    00    00   slice 0  6 - partsTimestamp length        if your string-format is something like  7 02 2014       use  var tstring   partsTimestamp slice 0  3  reverse   join  -    var tstring   partsTimestamp slice 0  3  join  -    tstring     T    partsTimestamp slice 3  join         Z     configure as needed var timestamp   Date parse tstring     Your browser should provide the same timestamp result as Date parse with    new Date tstring   getTime

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Until the 5th edition spec came out  the Date parse method was completely implementation dependent  new Date string  is equivalent to Date parse string  except the latter returns a number rather than a Date   In the 5th edition spec the requirement was added to support a simplified  and slightly incorrect  ISO-8601  also see What are valid Date Time Strings in JavaScript    But other than that  there was no requirement for what Date parse   new Date string  should accept other than that they had to accept whatever Date toString output  without saying what that was    As of ECMAScript 2017  edition 8   implementations were required to parse their output for Date toString and Date toUTCString  but the format of those strings was not specified   As of ECMAScript 2019  edition 9  the format for Date toString and Date toUTCString  have been specified as  respectively     ddd MMM DD YYYY HH mm ss ZZ   timezone name  e g  Tue Jul 10 2018 18 39 58 GMT 0530  IST  ddd  DD MMM YYYY HH mm ss Ze g  Tue 10 Jul 2018 13 09 58 GMT   providing 2 more formats that Date parse should parse reliably in new implementations  noting that support is not ubiquitous and non   compliant implementations will remain in use for some time    I would recommend that date strings are parsed manually and the Date constructor used with year  month and day arguments to avoid ambiguity      parse a date in yyyy-mm-dd format function parseDate input       let parts   input split  -          new Date year  month    day    hours   minutes   seconds   ms         return new Date parts 0   parts 1 -1  parts 2       Note  months are 0-based

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According to http   blog dygraphs com 2012 03 javascript-and-dates-what-mess html the format  yyyy mm dd  solves the usual problems   He says   Stick to  YYYY MM DD  for your date strings whenever possible  It s universally supported and unambiguous  With this format  all times are local   I ve set tests  http   jsfiddle net jlanus ND2Qg 432  This format      avoids the day and month order ambiguity by using y m d ordering and a 4-digit year    avoids the UTC vs  local issue not complying with ISO format by using slashes    danvk  the dygraphs guy  says that this format is good in all browsers

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During recent experience writing a JS interpreter I wrestled plenty with the inner workings of ECMA JS dates  So  I figure I ll throw in my 2 cents here  Hopefully sharing this stuff will help others with any questions about the differences among browsers in how they handle dates   The Input Side  All implementations store their date values internally as 64-bit numbers that represent the number of milliseconds  ms  since 1970-01-01 UTC  GMT is the same thing as UTC   This date is the ECMAScript epoch that is also used by other languages such as Java and POSIX systems such as UNIX  Dates occurring after the epoch are positive numbers and dates prior are negative   The following code is interpreted as the same date in all current browsers  but with the local timezone offset   Date parse  1 1 1970       1 January  1970   In my timezone  EST  which is -05 00   the result is 18000000 because that s how many ms are in 5 hours  it s only 4 hours during daylight savings months   The value will be different in different time zones  This behaviour is specified in ECMA-262 so all browsers do it the same way   While there is some variance in the input string formats that the major browsers will parse as dates  they essentially interpret them the same as far as time zones and daylight saving is concerned even though parsing is largely implementation dependent   However  the ISO 8601 format is different  It s one of only two formats outlined in ECMAScript 2015  ed 6  specifically that must be parsed the same way by all implementations  the other is the format specified for Date prototype toString    But  even for ISO 8601 format strings  some implementations get it wrong  Here is a comparison output of Chrome and Firefox when this answer was originally written for 1 1 1970  the epoch  on my machine using ISO 8601 format strings that should be parsed to exactly the same value in all implementations   Date parse  1970-01-01T00 00 00Z             Chrome  0         FF  0 Date parse  1970-01-01T00 00 00-0500         Chrome  18000000  FF  18000000 Date parse  1970-01-01T00 00 00              Chrome  0         FF  18000000    In the first case  the  Z  specifier indicates that the input is in UTC time so is not offset from the epoch and the result is 0 In the second case  the  -0500  specifier indicates that the input is in GMT-05 00 and both browsers interpret the input as being in the -05 00 timezone  That means that the UTC value is offset from the epoch  which means adding 18000000ms to the date s internal time value  The third case  where there is no specifier  should be treated as local for the host system  FF correctly treats the input as local time while Chrome treats it as UTC  so producing different time values  For me this creates a 5 hour difference in the stored value  which is problematic  Other systems with different offsets will get different results    This difference has been fixed as of 2020  but other quirks exist between browsers when parsing ISO 8601 format strings   But it gets worse  A quirk of ECMA-262 is that the ISO 8601 date   only format  YYYY-MM-DD  is required to be parsed as UTC  whereas ISO 8601 requires it to be parsed as local  Here is the output from FF with the long and short ISO date formats with no time zone specifier   Date parse  1970-01-01T00 00 00             18000000 Date parse  1970-01-01                      0   So the first is parsed as local because it s ISO 8601 date and time with no timezone  and the second is parsed as UTC because it s ISO 8601 date only   So  to answer the original question directly   YYYY-MM-DD  is required by ECMA-262 to be interpreted as UTC  while the other is interpreted as local  That s why   This doesn t produce equivalent results   console log new Date Date parse  Jul 8  2005    toString        Local console log new Date Date parse  2005-07-08    toString         UTC   This does   console log new Date Date parse  Jul 8  2005    toString     console log new Date Date parse  2005-07-08T00 00 00    toString       The bottom line is this for parsing date strings  The ONLY ISO 8601 string that you can safely parse across browsers is the long form with an offset  either   HH mm or  Z    If you do that you can safely go back and forth between local and UTC time   This works across browsers  after IE9    console log new Date Date parse  2005-07-08T00 00 00Z    toString       Most current browsers do treat the other input formats equally  including the frequently used  1 1 1970   M D YYYY  and  1 1 1970 00 00 00 AM   M D YYYY hh mm ss ap  formats  All of the following formats  except the last  are treated as local time input in all browsers  The output of this code is the same in all browsers in my timezone  The last one is treated as -05 00 regardless of the host timezone because the offset is set in the timestamp   console log Date parse  1 1 1970     console log Date parse  1 1 1970 12 00 00 AM     console log Date parse  Thu Jan 01 1970     console log Date parse  Thu Jan 01 1970 00 00 00     console log Date parse  Thu Jan 01 1970 00 00 00 GMT-0500       However  since parsing of even the formats specified in ECMA-262 is not consistent  it is recommended to never rely on the built   in parser and to always manually parse strings  say using a library and provide the format to the parser   E g  in moment js you might write   let m   moment  1 1 1970    M D YYYY       The Output Side  On the output side  all browsers translate time zones the same way but they handle the string formats differently  Here are the toString functions and what they output  Notice the toUTCString and toISOString functions output 5 00 AM on my machine  Also  the timezone name may be an abbreviation and may be different in different implementations    Converts from UTC to Local time before printing   - toString  - toDateString  - toTimeString  - toLocaleString  - toLocaleDateString  - toLocaleTimeString   Prints the stored UTC time directly   - toUTCString  - toISOString      In Chrome  toString            Thu Jan 01 1970 00 00 00 GMT-05 00  Eastern Standard Time  toDateString        Thu Jan 01 1970 toTimeString        00 00 00 GMT-05 00  Eastern Standard Time  toLocaleString      1 1 1970 12 00 00 AM toLocaleDateString  1 1 1970 toLocaleTimeString  00 00 00 AM  toUTCString         Thu  01 Jan 1970 05 00 00 GMT toISOString         1970-01-01T05 00 00 000Z     In Firefox  toString            Thu Jan 01 1970 00 00 00 GMT-05 00  Eastern Standard Time  toDateString        Thu Jan 01 1970 toTimeString        00 00 00 GMT-0500  Eastern Standard Time  toLocaleString      Thursday  January 01  1970 12 00 00 AM toLocaleDateString  Thursday  January 01  1970 toLocaleTimeString  12 00 00 AM  toUTCString         Thu  01 Jan 1970 05 00 00 GMT toISOString         1970-01-01T05 00 00 000Z     I normally don t use the ISO format for string input  The only time that using that format is beneficial to me is when dates need to be sorted as strings  The ISO format is sortable as-is while the others are not  If you have to have cross-browser compatibility  either specify the timezone or use a compatible string format   The code new Date  12 4 2013   toString   goes through the following internal pseudo-transformation      12 4 2013  - gt  toUCT - gt   storage  - gt  toLocal - gt  print  12 4 2013    I hope this answer was helpful

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Another solution is to build an associative array with date format and then reformat data   This method is useful for date formatted in an unussual way   An example       mydate  01 02 12 10 20 43       myformat  dd mm yy HH MM ss         dtsplit mydate split                 dfsplit myformat split                     creates assoc array for date     df   new Array        for dc 0 dc lt 6 dc                  df dfsplit dc   dtsplit dc                         uses assc array for standard mysql format     dstring r     20  df  yy    -  df  mm    -  df  dd        dstring r         df  HH       df  MM       df  ss

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This light weight date parsing library should solve all similar problems  I like the library because it is quite easy to extend  It s also possible to i18n it  not very straight forward  but not that hard    Parsing example   var caseOne   Date parseDate  Jul 8  2005    M d  Y    var caseTwo   Date parseDate  2005-07-08    Y-m-d      And formatting back to string  you will notice both cases give exactly the same result    console log  caseOne dateFormat  M d  Y      console log  caseTwo dateFormat  M d  Y      console log  caseOne dateFormat  Y-m-d      console log  caseTwo dateFormat  Y-m-d

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Use moment js to parse dates   var caseOne   moment  Jul 8  2005    MMM D  YYYY   true  toDate    var caseTwo   moment  2005-07-08    YYYY-MM-DD   true  toDate      The 3rd argument determines strict parsing  available as of 2 3 0   Without it moment js may also give incorrect results

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The accepted answer from CMS is correct  I have just added some features     trim and clean input spaces parse slashes  dashes  colons and spaces has default day and time        parse a date time that can contains spaces  dashes  slashes  colons function parseDate input           trimes and remove multiple spaces and split by expected characters     var parts   input trim   replace          g     split    s-              new Date year  month    day    hours   minutes   seconds   ms           return new Date parts 0   parts 1 -1  parts 2     1  parts 3     0  parts 4     0  parts 5     0      Note  months are 0-based

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Both are correct  but they are being interpreted as dates with two different timezones  So you compared apples and oranges      local dates new Date  Jul 8  2005   toISOString                  2005-07-08T07 00 00 000Z  new Date  2005-07-08T00 00-07 00   toISOString       2005-07-08T07 00 00 000Z     UTC dates new Date  Jul 8  2005 UTC   toISOString              2005-07-08T00 00 00 000Z  new Date  2005-07-08   toISOString                   2005-07-08T00 00 00 000Z    I removed the Date parse   call since it s used automatically on a string argument  I also compared the dates using ISO8601 format so you could visually compare the dates between your local dates and the UTC dates  The times are 7 hours apart  which is the timezone difference and why your tests showed two different dates   The other way of creating these same local UTC dates would be   new Date 2005  7-1  8                2005-07-08T07 00 00 000Z  new Date Date UTC 2005  7-1  8       2005-07-08T00 00 00 000Z    But I still strongly recommend Moment js which is as simple yet powerful      parse string moment  2005-07-08   format             2005-07-08T00 00 00 02 00  moment utc  2005-07-08   format         2005-07-08T00 00 00Z     year  month  day  etc  moment  2005  7-1  8   format           2005-07-08T00 00 00 02 00  moment utc  2005  7-1  8   format       2005-07-08T00 00 00Z

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While CMS is correct that passing strings into the parse method is generally unsafe  the new ECMA-262 5th Edition  aka ES5  specification in section 15 9 4 2 suggests that Date parse   actually should handle ISO-formatted dates   The old specification made no such claim   Of course  old browsers and some current browsers still do not provide this ES5 functionality   Your second example isn t wrong   It is the specified date in UTC  as implied by Date prototype toISOString    but is represented in your local timezone

[javascript] Why does Date.parse give incorrect results?

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