I setup a UTC+0 time, and want it to be shown in different puters in each puter's local time zone. But when I run the code in Chrome in my puter(VMware VM), it gave me the following information:

Date.UTC(2014, 0, 27, 6) // 1390802400000
var now = new Date()               // Tue Aug 05 2014 07:32:38 GMT-0400 (EDT)
now.getTime()     // 1407238358829
new Date(1407238358829)  // Tue Aug 05 2014 07:32:38 GMT-0400 (EDT)
new Date(1390802400000)  // Mon Jan 27 2014 01:00:00 GMT-0500 (EST)

My puter is in GMT-4, but why the last line shows time of EST instead of EDT?

I also try the same code on other puters such as EC2 instance(GMT+0) and my friend's(GMT+8), these puters will only display time of their local time zone.

But why my puter display time of other timezone?

I setup a UTC+0 time, and want it to be shown in different puters in each puter's local time zone. But when I run the code in Chrome in my puter(VMware VM), it gave me the following information:

Date.UTC(2014, 0, 27, 6) // 1390802400000
var now = new Date()               // Tue Aug 05 2014 07:32:38 GMT-0400 (EDT)
now.getTime()     // 1407238358829
new Date(1407238358829)  // Tue Aug 05 2014 07:32:38 GMT-0400 (EDT)
new Date(1390802400000)  // Mon Jan 27 2014 01:00:00 GMT-0500 (EST)

My puter is in GMT-4, but why the last line shows time of EST instead of EDT?

I also try the same code on other puters such as EC2 instance(GMT+0) and my friend's(GMT+8), these puters will only display time of their local time zone.

But why my puter display time of other timezone?

• javascript
• timezone

3 Answers 3

The code is working properly. This is just how time zones actually work. Hopefully some proper terminology will clear up the confusion for you.

A "time zone offset" is not the same as a "time zone". You can read more about this in the section "Time zone != offset" in the timezone tag wiki.

You said:

My puter is in GMT-4

This is incorrect. Your puter is in the North American "Eastern Time" time zone. On Linux/Mac, this will likely show as "America/New_York". On windows it will show in your control panel as "Eastern Time (US & Canada)".

This time zone has a time zone offset of UTC-05:00 during the winter months, and UTC-04:00 during the summer months when daylight saving time is in effect. We call these "Eastern Standard Time" (or EST) and "Eastern Daylight Time" (or EDT) respectively. While sometimes these are referred to as "time zones", you may wish to consider them as "time zone segments" - as they refer to only a portion of the actual time zone.

To recap:

EST = Eastern Standard Time = UTC-05:00 = GMT-5
EDT = Eastern Daylight Time = UTC-04:00 = GMT-4

ET = Eastern Time = "America/New_York" = "Eastern Time (US & Canada)"
  - Uses EST in the winter
  - Uses EDT in the summer

new Date(1390802400000) shows as EST, because that value is in January, which is in the winter.

You also said:

I also try the same code on other puters such as EC2 instance(GMT+0) and my friend's(GMT+8), these puters will only display time of their local time zone.

The EC2 instance is likely configured for UTC. This is best practice for servers. That means it will always have a zero offset. It will not change for daylight saving time.

Your friend's puter is likely in a time zone that does not follow daylight saving time. For example (and I'm guessing here), perhaps he is in the "Asia/Shanghai" time zone, which is in use in China. Since China does not use daylight saving time, his offset is permanently UTC+08:00.

You can see a list of time zones and their offsets here.

Dates in Javascript are stored as UTC time, more specifically as milliseconds since a particular date (1/1/1970 00:00:00, to be exact). You can print them to local time using date.toLocaleString(). There are also specific methods for just the time and just the date: date.toLocaleDateString() and date.toLocaleTimeString(). These return a string because local time is intended to be displayed, not stored.

As a rule: ALWAYS store and process dates as UTC time until you're going to display them to the users. If you're reading local times from the user, you need to convert them to UTC time at the earliest possible moment and keep them as that until you display them again.

In your case, the reason why you get 1 date in EDT and 1 in EST is due to daylight savings time(setting your clock 1 hour ahead or back). For about half the year, certain parts of the US east coast (like New York) are in a different timezone than the other half of the year. In your case, August 5th and January 27th are effectively in different timezones for those that use DST. You effectively cannot have a time in January in EDT because that timezone only exists between somewhere in mid-March and Mid-Oktober.

Source: http://www.timeanddate./library/abbreviations/timezones/na/edt.html

When you do:

new Date(1390802400000)  // Mon Jan 27 2014 01:00:00 GMT-0500 (EST)

A new Date object is created with a time value of 1390802400000. If you simply write this to output (alert, console, whatever) then the default value provided is the same as Date.toString, which returns a string in a convenient format based on the local timezone offset.

If you want to see the Date as UTC, you can use the toISOString method:

var d = new Date(1390802400000);
console.log(d.toISOString());   // 2014-01-27T06:00:00.000Z

Or you can use the UTC methods (e.g. getUTCHours) and build a string in whatever format you like.

My puter is in GMT-4, but why the last line shows time of EST instead of EDT

Because on that date, your puter thinks the local timezone is EST (presumably US Eastern Standard Time, not Australian Eastern Standard Time). Is it?