Determine third Friday of the month given month and year

Question

Given a year and month, I'd like to determine the date of the third Friday of that month. How would I leverage moment.js to determine this?

E.g. October 2015 => 16th October 2015

Bordello

Answer 1

Given year and month as integers and assuming that Friday is the fifth day of the week in your locale (Monday is the first day of the week), you can have:

function getThirdFriday(year, month){
    // Convert date to moment (month 0-11)
    var myMonth = moment({year: year, month: month});
    // Get first Friday of the first week of the month
    var firstFriday = myMonth.weekday(4);
    var nWeeks = 2;
    // Check if first Friday is in the given month
    if( firstFriday.month() != month ){
        nWeeks++;
    }
    // Return 3rd Friday of the month formatted (custom format)
    return firstFriday.add(nWeeks, 'weeks').format("DD MMMM YYYY");
}

If you have month and year as a string, you can use moment parsing functions instead of the Object notation, so you will have:

var myMonth = moment("October 2015", "MMMM yyyy");

If Friday is not the fifth day of the week (day with index 4), you can get the correct index using moment.weekdays()

Won

According to momentjs.com/docs/#/get-set/day changing it from .weekday(4) (local aware) to simply .day(5) (local unaware) will fix any issues arising from different locales. – May

Answer 2

Based on on answers above (1 and 2),

I generalised the function so it can return any weekday or any week for any given date.

var getNthWeekday = function(baseDate, weekth, weekday){

    // parse base date
    var date = moment(baseDate);
    var year = date.year();
    var month = date.month();

    // Convert date to moment (month 0-11)
    var myMonth = moment({year: year, month: month});

    // assume we won't have to move forward any number of weeks
    var weeksToAdvance = weekth-1;

    // Get first weekday of the first week of the month
    var firstOccurranceOfDay = myMonth.weekday(weekday);

    // Check if first weekday occurrance is in the given month
    if( firstOccurranceOfDay.month() != month ){
        weeksToAdvance++;
    }

    // Return nth weekday of month formatted (custom format)
    return firstOccurranceOfDay.add(weeksToAdvance, 'weeks');

}

Rembrandt

Answer 3

Based on @VincenzoC.

This allows me to send in a moment and receive back a moment.

let getThirdFriday: function(mDate) {
  // Based on https://stackoverflow.com/a/34278588

  // By default we will need to add two weeks to the first friday
  let nWeeks = 2,
  month = mDate.month();

  // Get first Friday of the first week of the month
  mDate = mDate.date(1).day(5);

  // Check if first Friday is in the given month
  //  it may have gone to the previous month
  if (mDate.month() != month) {
    nWeeks++;
  }

  // Return 3rd Friday of the month formatted (custom format)
  return mDate.add(nWeeks, 'weeks');
}

Then I can call it like:

let threeMonth = getThirdFriday(
                                  moment()
                                  .add(3, 'months')
                         ).format("YYYY-MM-DD");

May