Elegant method to generate array of random dates within two dates

Question

I have a datepicker where I show two months and I want to randomly choose 3 dates in each visible month

  $('.date').datepicker({
    minDate: new Date(),
    dateFormat: 'DD, MM, d, yy',
    constrainInput: true,
    beforeShowDay: processDates,
    numberOfMonths: 2,
    showButtonPanel: true,
    showOn: "button",
    buttonImage: "images/calendar_icon.jpg",
    buttonImageOnly: true    
  });

Here is my calculation

var now = new Date();
var nowTime = parseInt(now.getTime()/1000);
var randomDateSet = {};

function getRandomSet(y,m) {
  var monthIndex = "m"+y+""+m; // m20121 for Jan
  if (randomDateSet[monthIndex]) return randomDateSet[monthIndex];
  // generate here
.
. - I need this part
.
  return randomDateSet[monthIndex];
}

function processDay(date) { // this is calculated for each day so we need a singleton for the array
  var dateTime = parseInt(date.getTime()/1000);
  if (dateTime <= (nowTime-86400)) {
    return [false]; // earlier than today
  }
  var m = date.getMonth(), d = date.getDate(), y = date.getFullYear();
  var randomDates = getRandomSet(y,m);

  for (i = 0; i < randomDates.length; i++) {
    if($.inArray((m+1) + '-' + d + '-' + y,randomDates) != -1 || new Date() > date) {
      return [true,"highlight","Some message"];
    }
  }
  return [true,"normal"]; // ordinary day
}

score 328 · Accepted Answer · edited Jul 21 '21 at 21:39

328

Maybe I am missing something, but isn't this it?

function randomDate(start, end) {
  return new Date(start.getTime() + Math.random() * (end.getTime() - start.getTime()));
}

const d = randomDate(new Date(2012, 0, 1), new Date());
console.log(d);

edited Jul 21 '21 at 21:39

deb

6,671
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answered Jan 27 '12 at 15:29

Tomasz Nurkiewicz

334,321
69
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674

1

Perhaps - can you do that for 3 dates without overlap in each month? – mplungjan Jan 27 '12 at 15:46
1

By *overlaps* do you mean generating the same date? No, this code does **not** handle this case, theoretically it can generate the same date (especially when you truncate time). But it shouldn't be hard to get rid of duplicates. – Tomasz Nurkiewicz Jan 27 '12 at 15:50
3

Related function for getting a date a number of days before (or after) the given date: `function randomDateAfterDate(start, days) { return new Date(start.getTime() + (Math.random()*days*24*60*60*1000)); }` – Autumn Leonard Nov 17 '15 at 15:29
to test @AutumnLeonard example for the case when we need the number of days before the given date we can use minus sign - randomDateAfterDate(new Date(2021, 4, 5), -365) – SAndriy May 05 '21 at 10:22

score 56 · Answer 2 · edited Jul 18 '22 at 07:28

56

console.log(
  new Date(new Date() - Math.random()*(1e+12))
)

edited Jul 18 '22 at 07:28

vsync

118,978
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answered Oct 06 '16 at 19:41

Demven Weir

807
7
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@Typescript users: `new Date(new Date().valueOf() - Math.random()*(1e+12))` – Page not found Jun 09 '23 at 13:53

score 10 · Answer 3 · answered Apr 24 '19 at 18:50

10

Using Moment.js && @Demven Weir's answer to get a string value like "03/02/1975".

moment(new Date(+(new Date()) - Math.floor(Math.random()*10000000000)))
.format('MM/DD/YYYY');

NOTE: Keep adding a zero at a time to increase the span of years produced.

answered Apr 24 '19 at 18:50

SoEzPz

14,958
8
61
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This adds nothing to the question. You're adding formatting that nobody asked for. – Page not found Jun 09 '23 at 13:55

score 2 · Answer 4 · answered May 17 '21 at 21:28

2

function generateRandomDate() {
return new Date(+(new Date()) - Math.floor(Math.random() * 10000000000));
}

(new generateRandomDate()).toLocaleDateString('en-US')

You will get a random date in US format 4/25/2021

See this Demo on jsfiddle

answered May 17 '21 at 21:28

Husam Ebish

4,893
2
22
38

This adds nothing to the question. You're adding formatting that nobody asked for. – Page not found Jun 09 '23 at 13:54

score 1 · Answer 5 · answered Jan 27 '12 at 15:28

1

You can convert the boundary dates to integers (Date.getTime()) and then use Math.random() to generate your random dates within given boundaries. Then go back to Date objects with Date.setTime().

answered Jan 27 '12 at 15:28

bububaba

2,840
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x times without repeat (here x=3) ? – mplungjan Jan 27 '12 at 15:45
A matter of adding a loop and an array of already generated dates to check against when generating new one. Or am I missing something? – bububaba Jan 27 '12 at 15:48

score 0 · Answer 6 · answered Oct 28 '21 at 20:36

You can use this snippet to get random date and format;

const emptyDate = new Date();
const randomDate = new Date();
const dateFormatter = Intl.DateTimeFormat('sv-SE');
const formattedRandomDate = dateFormatter.format(emptyDate.setDate(randomDate.getDate() - Math.floor(Math.random()*1000)));

score 0 · Answer 7 · edited Feb 10 '22 at 08:47

0

From examples in date-fns

const result = eachDayOfInterval({
  start: new Date(2014, 9, 6),
  end: new Date(2014, 9, 10)
})

edited Feb 10 '22 at 08:47

Super Kai - Kazuya Ito

22,221
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answered Feb 07 '22 at 08:44

Ultrahalf

1

Elegant method to generate array of random dates within two dates

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