Calculate the recurring dates between a range of dates in javascript

Question

How the recurrence actually works is not a question. I want to implement a way to calculate the days between two dates with a specified recurrence interval. That could be weekly, monthly, bi-monthly(which I don't exactly know about) yearly etc. The simplest thing I have done until now is the following which let me count all the days between two dates and then loop through them with an interval of seven days for weekly recurrence. I would be grateful if you can suggest me the better and correct implementation of it. Thanks.

   //Push in the selected dates in the selected array.
                for (var i = 1; i < between.length; i += 7) {
                    selected.push(between[i]);
                    console.log(between[i]);

                }

Answer 1

Does this do something like what you're expecting? It would require an explicit argument for the number of days in the interval:

// startDate: Date()
// endDate: Date()
// interval: Number() number of days between recurring dates
function recurringDates(startDate, endDate, interval) {
  // initialize date variable with start date
  var date = startDate;
  // create array to hold result dates
  var dates = [];

  // check for dates in range
  while ((date = addDays(date, interval)) < endDate) {
    // add new date to array
    dates.push(date);
  }

  // return result dates
  return dates;
}

function addDays(date, days) {
  var newDate = new Date(date);
  newDate.setDate(date.getDate() + days);
  return newDate;
}

var startDate = new Date(2015, 0, 1);
var endDate = new Date(2016, 0, 1);
var interval = 20;
console.log(recurringDates(startDate, endDate, interval));

Here's the example on JSFiddle.

Answer 2

This function gives a date for every [interval] and [intervalType] (e.g. every 1 month) between two dates. It can also correct dates in weekends, if necessary. Is that what you had in mind?

Here a jsFiddle demo.

function recurringDates(startDate, endDate, interval, intervalType, noweekends) {
    intervalType = intervalType || 'Date';
    var date = startDate;
    var recurrent = [];
    var setget = {set: 'set'+intervalType, get: 'get'+intervalType};

    while (date < endDate) {
      recurrent.push( noweekends ? noWeekend() : new Date(date) );
      date[setget.set](date[setget.get]()+interval);
    }
    
    // add 1 day for sunday, subtract one for saturday
    function noWeekend() {
        var add, currdate = new Date(date), day = date.getDay();
        if (~[6,0].indexOf(day)) {
          currdate.setDate(currdate.getDate() + (add = day == 6 ? -1 : 1));
        }
        return new Date(currdate);
    }

    return recurrent;
}

Answer 3

If all you want is the number of recurrences, the fastest (constant time regardless of the size of the date range) is to do the following.

1. Calculate the number of days in the date range. See formula below.

2. Determine how many recurrences can fit in that number of days. This can be done with a simple division and floor operation. For instance, if the date range has 100 days, and you want a weekly recurrence, the number of recurrences is Math.floor(100 / 7)

It will help if you set the start of your date range to be the date of the first recurrence.

If you want to get the actual dates and also want to do things like excluding weekends or holidays, you need to iterate over the date range as follows.

// psuedo-code
d = start_date;
interval_days = recurrence_days;
n = 0;
while(is_same_day_or_before(d, end_date)) {
  if(not_an_excluded_day(d)) {
    print(d);
    n++;
  }
  d = add_days(d, interval_days)
}
print("There are " + n + " recurrences");

This approach will let you do things like excluding weekends and holidays, if necessary.

You can implement is_same_day_or_before(d1,d2) with a simple comparison like d1 <= d2. If d1 and d2 can be in different time zones, then you want a more sophisticated check that accommodates for day light savings adjustments, etc.

The add_days function is more straightforward.

function add_days(d,n) {
  var d = new Date(d.getTime());
  d.setDate(d.getDate() + n);
  return d;
}

Calculating the number of dates between two (javascript) dates

The answer here and copied below for reference gives you a fast and accurate way of doing this regardless of how large the date range is.

var _MS_PER_DAY = 1000 * 60 * 60 * 24;

// a and b are javascript Date objects
function dateDiffInDays(a, b) {
  // Discard the time and time-zone information.
  var utc1 = Date.UTC(a.getFullYear(), a.getMonth(), a.getDate());
  var utc2 = Date.UTC(b.getFullYear(), b.getMonth(), b.getDate());

  return Math.floor((utc2 - utc1) / _MS_PER_DAY);
}

Answer 4

The code can be either implemented in your own logic like you did or can use libraries. I reused the library later.js for the recurrence functionality.

Answer 5

You can first calculate the difference in milliseconds and then show the millisecond difference in other difference format like this:

    Date.daysBetween = function( date1, date2 ) {
  //Get 1 day in milliseconds
  var one_day=1000*60*60*24;

  // Convert both dates to milliseconds
  var date1_ms = date1.getTime();
  var date2_ms = date2.getTime();

  // Calculate the difference in milliseconds
  var difference_ms = date2_ms - date1_ms;
  //take out milliseconds
  difference_ms = difference_ms/1000;
  var seconds = Math.floor(difference_ms % 60);
  difference_ms = difference_ms/60; 
  var minutes = Math.floor(difference_ms % 60);
  difference_ms = difference_ms/60; 
  var hours = Math.floor(difference_ms % 24);  
  var days = Math.floor(difference_ms/24);

  return days + ' days, ' + hours + ' hours, ' + minutes + ' minutes, and ' + seconds + ' seconds';
}

//Set the two dates
var y2k  = new Date(2000, 0, 1);
var Jan1st2010 = new Date(y2k.getYear() + 10, y2k.getMonth(), y2k.getDate());
var today= new Date();
//displays "Days from Wed Jan 01 0110 00:00:00 GMT-0500 (Eastern Standard Time) to Tue Dec 27 2011 12:14:02 GMT-0500 (Eastern Standard Time): 694686 days, 12 hours, 14 minutes, and 2 seconds"
console.log('Days from ' + Jan1st2010 + ' to ' + today + ': ' + Date.daysBetween(Jan1st2010, today));