Date and time

Let’s meet a new built-in object: Date. It stores the date, time and provides methods for date/time management.

For instance, we can use it to store creation/modification times, to measure time, or just to print out the current date.

Creation

To create a new Date object call new Date() with one of the following arguments:

new Date()

Without arguments – create a Date object for the current date and time:

let now = new Date();
alert( now ); // shows current date/time

new Date(milliseconds)

Create a Date object with the time equal to number of milliseconds (1/1000 of a second) passed after the Jan 1st of 1970 UTC+0.

// 0 means 01.01.1970 UTC+0
let Jan01_1970 = new Date(0);
alert( Jan01_1970 );

// now add 24 hours, get 02.01.1970 UTC+0
let Jan02_1970 = new Date(24 * 3600 * 1000);
alert( Jan02_1970 );

An integer number representing the number of milliseconds that has passed since the beginning of 1970 is called a timestamp.

It’s a lightweight numeric representation of a date. We can always create a date from a timestamp using new Date(timestamp) and convert the existing Date object to a timestamp using the date.getTime() method (see below).

Dates before 01.01.1970 have negative timestamps, e.g.:

// 31 Dec 1969
let Dec31_1969 = new Date(-24 * 3600 * 1000);
alert( Dec31_1969 );

new Date(datestring)

If there is a single argument, and it’s a string, then it is parsed automatically. The algorithm is the same as Date.parse uses, we’ll cover it later.

let date = new Date("2017-01-26");
alert(date);
// The time is not set, so it's assumed to be midnight GMT and
// is adjusted according to the timezone the code is run in
// So the result could be
// Thu Jan 26 2017 11:00:00 GMT+1100 (Australian Eastern Daylight Time)
// or
// Wed Jan 25 2017 16:00:00 GMT-0800 (Pacific Standard Time)

new Date(year, month, date, hours, minutes, seconds, ms)

Create the date with the given components in the local time zone. Only the first two arguments are obligatory.

• The year should have 4 digits. For compatibility, 2 digits are also accepted and considered 19xx, e.g. 98 is the same as 1998 here, but always using 4 digits is strongly encouraged.
• The month count starts with 0 (Jan), up to 11 (Dec).
• The date parameter is actually the day of month, if absent then 1 is assumed.
• If hours/minutes/seconds/ms is absent, they are assumed to be equal 0.

For instance:

new Date(2011, 0, 1, 0, 0, 0, 0); // 1 Jan 2011, 00:00:00
new Date(2011, 0, 1); // the same, hours etc are 0 by default

The maximal precision is 1 ms (1/1000 sec):

let date = new Date(2011, 0, 1, 2, 3, 4, 567);
alert( date ); // 1.01.2011, 02:03:04.567

Access date components

There are methods to access the year, month and so on from the Date object:

getFullYear()

Get the year (4 digits)

getMonth()

Get the month, from 0 to 11.

getDate()

Get the day of month, from 1 to 31, the name of the method does look a little bit strange.

getHours(), getMinutes(), getSeconds(), getMilliseconds()

Get the corresponding time components.

Additionally, we can get a day of week:

getDay()

Get the day of week, from 0 (Sunday) to 6 (Saturday). The first day is always Sunday, in some countries that’s not so, but can’t be changed.

All the methods above return the components relative to the local time zone.

There are also their UTC-counterparts, that return day, month, year and so on for the time zone UTC+0: getUTCFullYear(), getUTCMonth(), getUTCDay(). Just insert the "UTC" right after "get".

If your local time zone is shifted relative to UTC, then the code below shows different hours:

// current date
let date = new Date();

// the hour in your current time zone
alert( date.getHours() );

// the hour in UTC+0 time zone (London time without daylight savings)
alert( date.getUTCHours() );

Besides the given methods, there are two special ones that do not have a UTC-variant:

getTime()

Returns the timestamp for the date – a number of milliseconds passed from the January 1st of 1970 UTC+0.

getTimezoneOffset()

Returns the difference between UTC and the local time zone, in minutes:

// if you are in timezone UTC-1, outputs 60
// if you are in timezone UTC+3, outputs -180
alert( new Date().getTimezoneOffset() );

Setting date components

The following methods allow to set date/time components:

• setFullYear(year, [month], [date])
• setMonth(month, [date])
• setDate(date)
• setHours(hour, [min], [sec], [ms])
• setMinutes(min, [sec], [ms])
• setSeconds(sec, [ms])
• setMilliseconds(ms)
• setTime(milliseconds) (sets the whole date by milliseconds since 01.01.1970 UTC)

Every one of them except setTime() has a UTC-variant, for instance: setUTCHours().

As we can see, some methods can set multiple components at once, for example setHours. The components that are not mentioned are not modified.

For instance:

let today = new Date();

today.setHours(0);
alert(today); // still today, but the hour is changed to 0

today.setHours(0, 0, 0, 0);
alert(today); // still today, now 00:00:00 sharp.

Autocorrection

The autocorrection