Question

Is it possible in JavaScript to keep Date objects in different time zones, e.g.:

date1.toString();
>>> 2012-02-16T14:00+02:00

date2.toString();
>>> 2012-03-16T13:00+01:00

i.e. I have two date objects they reflect the same moment of the time but keep their information in different time zones.

Answer 1

No. Dates in JavaScript represent a moment in time; they do not store time zone information. You can then choose to display what time that represents in a particular timezone. (See the various methods like getHours()—current local time zone—versus getUTCHours().)

To display a time in a timezone other than UTC or the local you need to write (or use) a function that does a bit of math:

function offsetDate( date, hours ){
  return date.setUTCHours( date.getUTCHours() + hours );
}

Edit: You can choose to store a custom offset along with a date (as you can add custom properties to any JS object):

Date.prototype.withZone = function(){
  var o = new Date(this.getTime());                // Make a copy for mutating
  o.setUTCHours(o.getUTCHours() + (this.tz || 0)); // Move the UTC time

  // Return a custom formatted version of the date
  var offset = this.tz ? (this.tz<0 ? this.tz : ('+'+this.tz)) : 'Z';
  return o.customFormat('#YYYY#-#MMM#-#D# @ #h#:#mm##ampm# ('+offset+')');
}

// http://phrogz.net/JS/FormatDateTime_js.txt
Date.prototype.customFormat = function(formatString){
  var YYYY,YY,MMMM,MMM,MM,M,DDDD,DDD,DD,D,hhh,hh,h,mm,m,ss,s,ampm,AMPM,dMod,th;
  YY = ((YYYY=this.getUTCFullYear())+"").slice(-2);
  MM = (M=this.getUTCMonth()+1)<10?('0'+M):M;
  MMM = (MMMM=["January","February","March","April","May","June","July","August","September","October","November","December"][M-1]).substring(0,3);
  DD = (D=this.getUTCDate())<10?('0'+D):D;
  DDD = (DDDD=["Sunday","Monday","Tuesday","Wednesday","Thursday","Friday","Saturday"][this.getUTCDay()]).substring(0,3);
  th=(D>=10&&D<=20)?'th':((dMod=D%10)==1)?'st':(dMod==2)?'nd':(dMod==3)?'rd':'th';
  formatString = formatString.replace("#YYYY#",YYYY).replace("#YY#",YY).replace("#MMMM#",MMMM).replace("#MMM#",MMM).replace("#MM#",MM).replace("#M#",M).replace("#DDDD#",DDDD).replace("#DDD#",DDD).replace("#DD#",DD).replace("#D#",D).replace("#th#",th);
  h=(hhh=this.getUTCHours());
  if (h==0) h=24; if (h>12) h-=12;
  hh = h<10?('0'+h):h;
  AMPM=(ampm=hhh<12?'am':'pm').toUpperCase();
  mm=(m=this.getUTCMinutes())<10?('0'+m):m;
  ss=(s=this.getUTCSeconds())<10?('0'+s):s;
  return formatString.replace("#hhh#",hhh).replace("#hh#",hh).replace("#h#",h).replace("#mm#",mm).replace("#m#",m).replace("#ss#",ss).replace("#s#",s).replace("#ampm#",ampm).replace("#AMPM#",AMPM);
}

var now = new Date;            // Make a plain date
console.log( now.withZone() ); //-> 2012-Feb-16 @ 9:37pm (Z)
now.tz = -7;                   // Add a custom property for our method to use
console.log( now.withZone() ); //-> 2012-Feb-16 @ 2:37pm (-7)