Correct way to initializa Date in EcmaScript5

Question

What is the correct way to initialize Date in EcmaScript5.

Here is what I am trying to do:

//18.02.09 12:00 - 18.02.11 24:00
 var StartTime = new Date("2018-02-09T12:00:00.000Z").getTime();
 var EndTime = new Date("2018-02-11T23:59:00.000Z").getTime();

What am I doing wrong here?

I am changing the time on my laptop to 18.02.09 12:01 PM. Then I am calling new Date().now(). What I expect is to get a number which would be between StartTime and EndTime. But I am getting something less on two hours than StartTime.

Answer 1

Strings such as "2018-02-09T12:00:00.000Z" are a standardized way to define dates, given by ISO 8601.

There are various ways to construct a Date object in JavaScript. From the Date MDN docs, we can see that one of the ways of doing that is by providing a string to be parsed. There is even an example:

var date2 = new Date("1995-12-17T03:24:00");

But you should not do that. That same page warns that creating a Date by parsing a string is not recommended since it is implementation-dependant (i.e. it might work differently depending on the browser).

Regardless, it is important to mention that in ISO 8601, ending a timestamp with the letter "Z" has a special meaning - it says that instead of considering local time, the date should be interpreted as being in UTC. So, regardless of how you decide do construct your Date object, don't forget to decide whether you want your timestamps to be localized or referring to the UTC standard time.

A safe way to construct your Date object is by providing the arguments separately:

new Date(year, month);
new Date(year, month, day);
new Date(year, month, day, hours);
new Date(year, month, day, hours, minutes);
new Date(year, month, day, hours, minutes, seconds);
new Date(year, month, day, hours, minutes, seconds, milliseconds);

You can also consider using an awesome library called MomentJS, whose size is 16.3 kB at the time of this writing (definitely worth considering), which is very famous and widely used, with over 12 million downloads per month (at the time of this writing).

Answer 2

There are many ways to create a Date, either passing a single string, single integer or multiple arguments. The best way depends on your particular circumstance.

By far the most robust way is to use the Date constructor with discrete values. If you wish to create a date for 18.02.09 12:00 then assuming it is YY.MM.DD HH:mm use:

var d = new Date(2018, 1, 9, 12);

Which will create a date for 9 February, 2018 at 12 noon local time, where "local" is determined from the host system timezone offset.

Passing strings to the Date constructor is strongly recommended against for the reasons expressed in answers to Why does Date.parse give incorrect results?, so don't use the built-in parser, it's unreliable and you can get unexpected results.

The string "2018-02-09T12:00:00.000Z" will be treated as UTC, so the Date created from it will be offset by the host local offset. Using UTC time differences will be different to local time differences where the host timezone offset changes for the two dates, e.g. where a daylight saving boundary is crossed.

So the "correct" or "best" way depends greatly on what you are trying to do, the input values and the outcome you wish to achieve. The bottom line is never use the built-in parser, always manually parse strings either with a bespoke function or use library.