Just like most programming languages, JavaScript uses Unix time. This is actually a system for representing a given instance of time, for how many seconds or, in JavaScript's case, milliseconds have passed since midnight January 1, 1970 in Universal Coordinated Time, commonly known as UTC.

This number is actually not fully compliant with UTC, nor does it account for the various atypical situations such as leap seconds, but this is acceptable in most cases.

In JavaScript, we have the Date object that can be constructed in different ways:

new Date() // uses local time
new Date(someNumber) //create date with milliseconds since epoch
new Date(dateString) // create date from input string representation
new Date(year, month, day [, hour, minute, second, millisecond])

During construction, if you supply some of the arguments and leave out the optional ones, they get defaulted to zero. And one other thing to note is that months in JavaScript are zero based while days are not.

What we can usually do is return the millisecond representation from the server side and have the number formatted in the local time zone. So let's say that our API returned us the milliseconds 1355270400000 that is actually 12.12.2012, which is also known as the end-of-the-world date.

The creation of the date is as follows:

var endOfTheWorld= new Date(1355270400000);

When printing in the local string, there are few available options; one of them is toLocaleDateString:

   endOfTheWorld.toLocaleDateString()

This method uses the underlying operation system to get the formatting convention. For example, in the U.S. the format is month/day/year while in other countries it is day/month/year. For our case, the end of the world is on "Wednesday, December 12, 2012". You could also manually construct the printed date using the appropriate getX methods.

There is also a method of printing out the local time called toLocaleTimeString that can be used on our end-of-the-world date. Because this method also uses the operating system's local time for us, it is 01:00:00, because we are in the UTC+1 time zone. For us, this means that we have one extra hour to live; or maybe not?

In order to get the offset for the local user, there is a method in the Date object called getTimezoneOffset() that returns the time zone offset from the date to UTC in minutes. The problem is that there is no such method for hours and, additionally, it is contraintuitive as we usually want to know the difference from UTC to the given date.

If working with dates is something common that you need for your application, it makes sense to use a library, such as Moment.js (http://momentjs.com/).

Moment.js provides support for internationalization and the more advanced manipulation of dates. For example, removing 10 days from the current date can simply be accomplished with the following code:

moment().subtract('days', 10).calendar();

For getting the time from today's start of day, use the following code:

moment().startOf('day').fromNow();