How do I get the difference between two Dates in JavaScript

Question

I m creating an application which lets you define events with a time frame   I want to automatically fill in the end date when the user selects or changes the start date   I can t quite figure out  however  how to get the difference between the two times  and then how to create a new end Date using that difference

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function checkdate         var indate   new Date       indate setDate dat      indate setMonth mon - 1      indate setFullYear year       var one day   1000   60   60   24     var diff   Math ceil  indate getTime   - now getTime       one day       var str   diff     days are remaining        document getElementById  print   innerHTML   str fontcolor  blue

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function compare       var end actual time          date3   val       start actual time   new Date      end actual time   new Date end actual time      var diff   end actual time-start actual time     var diffSeconds   diff 1000    var HH   Math floor diffSeconds 3600     var MM   Math floor diffSeconds 3600  60     var formatted     HH  lt  10    0    HH  HH            MM  lt  10    0    MM  MM    getTime diffSeconds     function getTime seconds      var days   Math floor leftover   86400        how many seconds are left   leftover   leftover -  days   86400        how many full hours fits in the amount of leftover seconds   var hours   Math floor leftover   3600        how many seconds are left   leftover   leftover -  hours   3600        how many minutes fits in the amount of leftover seconds   var minutes   leftover   60       how many seconds are left     leftover   leftover -  minutes   60     alert days         hours         minutes

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this code fills the duration of study years when you input the start date and end date qualify accured date  of study and check if the duration less than a year if yes the alert a message  take in mind there are three input elements the first txtFromQualifDate and second txtQualifDate and third txtStudyYears  it will show result of number of years with fraction  function getStudyYears                 if document getElementById  txtFromQualifDate   value        amp  amp  document getElementById  txtQualifDate   value                              var d1   document getElementById  txtFromQualifDate   value               var d2   document getElementById  txtQualifDate   value               var one day 1000 60 60 24               var x   d1 split                   var y   d2 split                    var date1 new Date x 2   x 1 -1  x 0                 var date2 new Date y 2   y 1 -1  y 0                var dDays    date2 getTime  -date1 getTime    one day               if dDays  lt  365                                alert  the date between start study and graduate must not be less than a year                       document getElementById  txtQualifDate   value                       document getElementById  txtStudyYears   value                        return                              var dMonths   Math ceil dDays   30                var dYears   Math floor dMonths  12          dMonths   12               document getElementById  txtStudyYears   value   dYears

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If you don t care about the time component  you can use  getDate   and  setDate   to just set the date part   So to set your end date to 2 weeks after your start date  do something like this   function GetEndDate startDate        var endDate   new Date startDate getTime         endDate setDate endDate getDate   14       return endDate      To return the difference  in days  between two dates  do this   function GetDateDiff startDate  endDate        return endDate getDate   - startDate getDate        Finally  let s modify the first function so it can take the value returned by 2nd as a parameter   function GetEndDate startDate  days        var endDate   new Date startDate getTime         endDate setDate endDate getDate     days       return endDate

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Depending on your needs  this function will calculate the difference between the 2 days  and return a result in days decimal      This one returns a signed decimal  The sign indicates past or future   this getDateDiff   function date1  date2        return  date1 getTime   - date2 getTime       1000   60   60   24         This one always returns a positive decimal   Suggested by Koen below   this getDateDiff   function date1  date2        return Math abs  date1 getTime   - date2 getTime       1000   60   60   24

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var getDaysLeft   function  date1  date2       var daysDiffInMilliSec   Math abs new Date date1  - new Date date2       var daysLeft   daysDiffInMilliSec    1000   60   60   24         return daysLeft     var date1  2018-05-18   var date2  2018-05-25   var dateDiff   getDaysLeft date1  date2   console log dateDiff

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alternative modificitaion extended code    http   jsfiddle net vvGPQ 48   showDiff     function showDiff    var date1   new Date  2013 01 18 06 59 00       var date2   new Date      Customise date2 for your required future time  var diff    date2 - date1  1000  var diff   Math abs Math floor diff     var years   Math floor diff  365 24 60 60    var leftSec   diff - years   365 24 60 60   var month   Math floor leftSec   365 12  24 60 60    var leftSec   leftSec - month    365 12  24 60 60       var days   Math floor leftSec  24 60 60    var leftSec   leftSec - days   24 60 60   var hrs   Math floor leftSec  60 60    var leftSec   leftSec - hrs   60 60   var min   Math floor leftSec  60    var leftSec   leftSec - min   60      document getElementById  showTime   innerHTML    You have     years     years    month     month     days     days     hrs     hours     min     minutes and     leftSec     seconds the life time has passed     setTimeout showDiff 1000

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See JsFiddle DEMO      var date1   new Date            var date2   new Date  2025 07 30 21 59 00          Customise date2 for your required future time      showDiff     function showDiff date1  date2        var diff    date2 - date1  1000      diff   Math abs Math floor diff         var days   Math floor diff  24 60 60        var leftSec   diff - days   24 60 60       var hrs   Math floor leftSec  60 60        var leftSec   leftSec - hrs   60 60       var min   Math floor leftSec  60        var leftSec   leftSec - min   60       document getElementById  showTime   innerHTML    You have     days     days     hrs     hours     min     minutes and     leftSec     seconds before death     setTimeout showDiff 1000       for your HTML Code    lt div id  showTime  gt  lt  div gt

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lt html gt   lt head gt   lt script gt  function dayDiff          var start   document getElementById  datepicker   value       var end  document getElementById  date picker   value       var oneDay   24 60 60 1000        var firstDate   new Date start        var secondDate   new Date end            var diffDays   Math round Math abs  firstDate getTime   - secondDate getTime     oneDay         document getElementById  leave   value  diffDays       lt  script gt   lt  head gt   lt body gt   lt input type  text  name  datepicker value     gt   lt input type  text  name  date picker  onclick  function dayDiff    value     gt   lt input type  text  name  leave  value     gt   lt  body gt   lt  html gt

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Thanks  Vincent Robert  I ended up using your basic example  though it s actually newBegin   oldEnd - oldBegin   Here s the simplified end solution          don t update end date if there s already an end date but not an old start date     if   oldEnd    oldBegin            var selectedDateSpan   1800000     30 minutes         if  oldEnd                selectedDateSpan   oldEnd - oldBegin                    newEnd   new Date newBegin getTime     selectedDateSpan

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In JavaScript  dates can be transformed to the number of milliseconds since the epoc by calling the getTime   method or just using the date in a numeric expression   So to get the difference  just subtract the two dates   To create a new date based on the difference  just pass the number of milliseconds in the constructor   var oldBegin       var oldEnd       var newBegin        var newEnd   new Date newBegin   oldEnd - oldBegin     This should just work  EDIT  Fixed bug pointed by  bdukes  EDIT    For an explanation of the behavior  oldBegin  oldEnd  and newBegin are Date instances  Calling operators   and - will trigger Javascript auto casting and will automatically call the valueOf   prototype method of those objects  It happens that the valueOf   method is implemented in the Date object as a call to getTime     So basically  date getTime       date valueOf        0   date        date

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JavaScript perfectly supports date difference out of the box https   jsfiddle net b9chris v5twbe3h  var msMinute   60 1000       msDay   60 60 24 1000      a   new Date 2012  2  12  23  59  59       b   new Date  quot 2013 march 12 quot      console log Math floor  b - a    msDay      full days between       364 console log Math floor   b - a    msDay    msMinute      full minutes between       0   Now some pitfalls  Try this  console log a - 10      1331614798990 console log a   10      mixed string  So if you have risk of adding a number and Date  convert Date to number directly  console log a getTime   - 10      1331614798990 console log a getTime     10      1331614799010  My fist example demonstrates the power of Date object but it actually appears to be a time bomb

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Below code  will return the days left from today to futures date   Dependencies  jQuery and MomentJs   var getDaysLeft   function  date      var today   new Date      var daysLeftInMilliSec   Math abs new Date moment today  format  YYYY-MM-DD    - new Date date      var daysLeft   daysLeftInMilliSec    1000   60   60   24        return daysLeft      getDaysLeft  YYYY-MM-DD

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If you use Date objects and then use the getTime   function for both dates it will give you their respective times since Jan 1  1970 in a number value  You can then get the difference between these numbers    If that doesn t help you out  check out the complete documentation  http   www w3schools com jsref jsref obj date asp

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If using moment js  there is a simpler solution  which will give you the difference in days in one single line of code   moment endDate  diff moment beginDate    days      Additional details can be found in the moment js page  Cheers  Miguel

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THIS IS WHAT I DID ON MY SYSTEM    var startTime   08 00 00   split       var endTime   16 00 00   split       var HoursInMinutes   parseInt endTime 0   60  parseInt endTime 1   -  parseInt startTime 0   60  parseInt startTime 1     console log HoursInMinutes 60

[javascript] How do I get the difference between two Dates in JavaScript?

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