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
}

deb · Accepted Answer · 2021-07-21 21:39:30Z

343

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, 2021 at 21:39

deb

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answered Jan 27, 2012 at 15:29

Tomasz Nurkiewicz

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4 Comments

mplungjan Over a year ago

Perhaps - can you do that for 3 dates without overlap in each month?

Tomasz Nurkiewicz Over a year ago

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.

Autumn Leonard Over a year ago

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));     }

SAndriy Over a year ago

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)

vsync · Accepted Answer · 2022-07-18 07:28:33Z

66

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

edited Jul 18, 2022 at 7:28

vsync

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answered Oct 6, 2016 at 19:41

Demven Weir

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1 Comment

Page not found Over a year ago

@Typescript users: new Date(new Date().valueOf() - Math.random()*(1e+12))

SoEzPz · Accepted Answer · 2019-04-24 18:50:54Z

13

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, 2019 at 18:50

SoEzPz

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1 Comment

Page not found Over a year ago

This adds nothing to the question. You're adding formatting that nobody asked for.

Husam Ebish · Accepted Answer · 2021-05-17 21:28:05Z

3

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, 2021 at 21:28

Husam Ebish

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1 Comment

Page not found Over a year ago

This adds nothing to the question. You're adding formatting that nobody asked for.

bububaba · Accepted Answer · 2012-01-27 15:28:41Z

2

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, 2012 at 15:28

bububaba

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2 Comments

mplungjan Over a year ago

x times without repeat (here x=3) ?

bububaba Over a year ago

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?

Mahir Uslu · Accepted Answer · 2021-10-28 20:36:22Z

2

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)));

answered Oct 28, 2021 at 20:36

Mahir Uslu

414 bronze badges

Comments

Super Kai - Kazuya Ito · Accepted Answer · 2022-02-10 08:47:20Z

2

From examples in date-fns

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

edited Feb 10, 2022 at 8:47

Super Kai - Kazuya Ito

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answered Feb 7, 2022 at 8:44

Ultrahalf

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1 Comment

Kalnode Over a year ago

Isn't this missing randomness?

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