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glumn/weekDatesCalculator.js

Last active August 29, 2015 14:22
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  1. Guillaume Mantopoulos renamed this gist Aug 6, 2015. 1 changed file with 0 additions and 0 deletions.
    0 gistfile1.js → weekDatesCalculator.js
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  2. Guillaume Mantopoulos revised this gist Aug 6, 2015. No changes.
  3. Guillaume Mantopoulos created this gist Jun 2, 2015.
    53 changes: 53 additions & 0 deletions gistfile1.js
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    // http://en.wikipedia.org/wiki/ISO_8601#Week_dates
    // http://stackoverflow.com/questions/662379/calculate-date-from-week-number
    getDatesFromWeek: function (week, year, startOfWeekDay) {
    if (startOfWeekDay == undefined || startOfWeekDay == null)
    startOfWeekDay = 0; // Default StartOfWeek to Sunday if not provided

    // ISO 8601 defines the week where first Thursday of the year is has Week 1
    // create a variable holding the position of thursday in the week
    var thursday = 4;

    // First set a basis of january one for the current year
    var januaryOne = new Date(Date.UTC(year, 0, 1));

    // ISO 8601 defines the week where first Thursday of the year is has Week 1
    // Calculate the offset of days between the first thursday and January 1
    var daysOffset = thursday - januaryOne.getDay();

    // Then get the first thursday date
    var firstThursday = januaryOne;
    firstThursday.setDate(firstThursday.getDate() + daysOffset);

    // Later on we will multiply the week we ask for
    // by 7 days to add to the date, so lower week to 0 if we're asking for first week
    var firstWeek = moment(firstThursday).isoWeek();
    var weekNumber = week;
    if (firstWeek <= 1) {
    weekNumber -= 1;
    }

    // Match week that was asked for
    // Add as much days as needed depending on the week we're asking for
    var startDate = firstThursday;
    startDate.setDate(startDate.getDate() + (weekNumber * 7));

    // Start Date right now is set to thursday, match the start of week day that was asked for by adding / removing days
    var startDateDayOffset;
    if (startOfWeekDay == 0) {
    // If specific start of day is Sunday, add 3 days
    startDateDayOffset = 3;
    } else {
    // Otherwise the offset calculation is simple
    startDateDayOffset = startOfWeekDay - thursday;
    }

    // Apply the calculated offset to get the start date for the provided week given a specific starting day
    startDate.setDate(startDate.getDate() + startDateDayOffset);

    // End Date is startDate + 6
    var endDate = new Date(Date.UTC(startDate.getFullYear(), startDate.getMonth(), startDate.getDate() + 6));

    return { startDate: startDate, endDate: endDate, week: week, year: year };
    },