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.
create a new
Date object call
new Date() with one of the following arguments:
Without arguments – create a
Date object for the current date and time:
let now = new Date(); alert( now ); // shows current date/time
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 );
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 );
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.
yearshould have 4 digits. For compatibility, 2 digits are also accepted and considered
98is the same as
1998here, but always using 4 digits is strongly encouraged.
monthcount starts with
0(Jan), up to
dateparameter is actually the day of month, if absent then
hours/minutes/seconds/msis absent, they are assumed to be equal
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
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.
getYear(). This method is deprecated. It returns 2-digit year sometimes. Please never use it. There is
getFullYear() for the year.
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
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:
Returns the timestamp for the date – a number of milliseconds passed from the January 1st
of 1970 UTC+0.
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
following methods allow to set date/time components:
setFullYear(year, [month], [date])
setHours(hour, [min], [sec], [ms])
setMinutes(min, [sec], [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:
As we can see, some methods can set multiple components at once,
setHours. The components that are not mentioned are not modified.
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.
The autocorrection is a very handy feature of
Date objects. We can set out-of-range values, and it will auto-adjust itself.
let date = new Date(2013, 0, 32); // 32 Jan 2013 ?!? alert(date); // ...is 1st Feb 2013!
Out-of-range date components are distributed automatically.
Let’s say we need to increase the date “28 Feb 2016” by 2 days. It may be “2 Mar” or “1 Mar” in case of a leap-year. We don’t need to think about it. Just add 2 days. The
Date object will do the rest:
let date = new Date(2016, 1, 28); date.setDate(date.getDate() + 2); alert( date ); // 1 Mar 2016
That feature is often used to get the date after the given period of time. For instance, let’s get the date for “70 seconds after now”:
let date = new Date(); date.setSeconds(date.getSeconds() + 70); alert( date ); // shows the correct date
We can also set zero or even negative values. For example:
let date = new Date(2016, 0, 2); // 2 Jan 2016 date.setDate(1); // set day 1 of month alert( date ); date.setDate(0); // min day is 1, so the last day of the previous month is assumed alert( date ); // 31 Dec 2015
Date to number, date
Date object is converted to number, it becomes the timestamp same as
let date = new Date(); alert(+date); // the number of milliseconds, same as date.getTime()
The important side effect: dates can be subtracted, the result is their difference in ms.
That can be used for time measurements:
let start = new Date(); // start measuring time // do the job for (let i = 0; i < 100000; i++) let doSomething = i * i * i; let end = new Date(); // end measuring time alert( `The loop took $end - start ms` );
If we only
want to measure time, we don’t need the
There’s a special method
Date.now() that returns the current timestamp.
It is semantically equivalent to
new Date().getTime(), but it doesn’t create an intermediate
Date object. So it’s faster and doesn’t put pressure on garbage collection.
So this is probably better:
let start = Date.now(); // milliseconds count from 1 Jan 1970 // do the job for (let i = 0; i < 100000; i++) let doSomething = i * i * i; let end = Date.now(); // done alert( `The loop took $end - start ms` ); // subtract numbers, not dates
If we want a reliable benchmark of CPU-hungry function, we should be careful.
For instance, let’s measure two functions that calculate the difference between two dates: which one is faster?
Such performance measurements are often
// we have date1 and date2, which function faster returns their difference in ms? function diffSubtract(date1, date2) return date2 - date1; // or function diffGetTime(date1, date2) return date2.getTime() - date1.getTime();
These two do exactly the same thing, but one of them uses an explicit
date.getTime() to get the date in ms, and the other one relies on a date-to-number transform. Their result is always the same.
So, which one is faster?
The first idea may be to run them many times in a row and measure the time difference. For our case, functions are very simple, so we have to do it at least 100000 times.
function diffSubtract(date1, date2) return date2 - date1; function diffGetTime(date1, date2) return date2.getTime() - date1.getTime(); function bench(f) let date1 = new Date(0); let date2 = new Date(); let start = Date.now(); for (let i = 0; i < 100000; i++) f(date1, date2); return Date.now() - start; alert( 'Time of diffSubtract: ' + bench(diffSubtract) + 'ms' ); alert( 'Time of diffGetTime: ' + bench(diffGetTime) + 'ms' );
getTime() is so much faster! That’s because there’s no type conversion, it is much easier for engines to optimize.
Okay, we have something. But that’s not a good benchmark yet.
Imagine that at the time of running
bench(diffSubtract) CPU was doing something in parallel, and it was taking resources. And by the time of running
bench(diffGetTime) that work has finished.
A pretty real scenario for a modern
As a result, the first benchmark will have less CPU resources than the second. That may lead to wrong results.
For more reliable benchmarking, the whole pack of benchmarks should be rerun multiple times.
For example, like this:
function diffSubtract(date1, date2) return date2 - date1; function diffGetTime(date1, date2) return date2.getTime() - date1.getTime(); function bench(f) let date1 = new Date(0); let date2 = new Date(); let start = Date.now(); for (let i = 0; i < 100000; i++) f(date1, date2); return Date.now() - start; let time1 = 0; let time2 = 0; // run bench(diffSubtract) and bench(diffGetTime) each 10 times alternating for (let i = 0; i < 10; i++) time1 += bench(diffSubtract); time2 += bench(diffGetTime); alert( 'Total time for diffSubtract: ' + time1 ); alert( 'Total time for diffGetTime: ' + time2 );
executes many times (no need to optimize rarely executed things). So, in the example above, first executions are not well-optimized. We may want to add a heat-up run:
// added for "heating up" prior to the main loop bench(diffSubtract); bench(diffGetTime); // now benchmark for (let i = 0; i < 10; i++) time1 += bench(diffSubtract); time2 += bench(diffGetTime);
Be careful doing microbenchmarking
The great pack of articles about V8 can be found at http://mrale.ph.
Date.parse from a string
Date.parse(str) can read a date from a string.
The string format should be:
YYYY-MM-DD– is the date: year-month-day.
- The character
"T"is used as the delimiter.
HH:mm:ss.sss– is the time: hours, minutes, seconds and milliseconds.
- The optional
'Z'part denotes the time zone in the format
+-hh:mm. A single letter
Zwould mean UTC+0.
Shorter variants are also possible, like
YYYY-MM or even
The call to
Date.parse(str) parses the string in the given format and returns the timestamp (number of milliseconds from 1 Jan 1970 UTC+0). If the format is invalid, returns
let ms = Date.parse('2012-01-26T13:51:50.417-07:00'); alert(ms); // 1327611110417 (timestamp)
We can instantly create a
new Date object from the timestamp:
let date = new Date( Date.parse('2012-01-26T13:51:50.417-07:00') ); alert(date);
Dateobjects always carry both.
- Months are counted from zero (yes, January is a zero month).
- Days of week in
also counted from zero (that’s Sunday).
Dateauto-corrects itself when out-of-range components are set. Good for adding/subtracting days/months/hours.
- Dates can be subtracted, giving their difference in milliseconds. That’s because a
Datebecomes the timestamp when converted to a number.
Date.now()to get the current timestamp fast.
alert(`Loading started $performance.now()ms ago`); // Something like: "Loading started 34731.26000000001ms ago" // .26 is microseconds (260 microseconds) // more than 3 digits after the decimal point are precision errors, only the first 3 are correct
microtime module and other ways. Technically, almost any device and environment allows to get more precision, it’s just not in
How do I convert a date to a short date?
DateTimeFormatInfo format = new DateTimeFormatInfo(); format. ShortDatePattern = “yyyy-MM-dd”; format. DateSeparator = “-“; DateTime date = 2012-03-11 09:26:37.837; DateTime shortDate = Convert. ToDateTime(date , format);
Use the toLocaleDateString() method to convert a full date to a short date. The method takes the locale and an options object as parameters and returns a string that represents the date according to the provided locale and options. Copied! const date = new Date(‘2022-09-24’); // 👇️ “09/24/2022” console.
MM — month from 01 to 12..
MMM — month abbreviation from Jan. to Dec..
DD — day from 01 to last day of the month (various).
YYYY — year as a 4-digit number..