Calculating the week number

gistfile1.js

// Converts Su-Sa (0-6) to other periods, such as Mo-Su (0-6)
function weekDayWithOffset(date, offset) {
	return ((date.getDay() + 7 - offset) % 7);
}

// Calculates the week number of the given date
// Thanks to http://stackoverflow.com/a/6117889/316944
function weekNum(date) {
	var refDate = new Date(date.getTime());

	// Set the ref date to the week's thursday; this is due to the ISO-8601 standard that specifies
	// that the week number falls on the year that has the thursday of that week.
	refDate.setDate(refDate.getDate() + (4 - refDate.getDay()));

	// Jan 1st
	var startOfYear = new Date(date.getFullYear(), 0, 1);

	// Difference between the start of the year and the input date in days
	var dayDiff = ((date.getTime() - startOfYear.getTime()) / (1000 * 60 * 60 * 24));

	// The weekday of Jan 1st, with the week starting on Monday
	var weekDayOfStartOfYear = weekDayWithOffset(startOfYear, 1);
	// Handle the day difference as if the year had started on the first day of the week Jan 1st lands on
	dayDiff += weekDayOfStartOfYear;

	// We floor the day difference and add 1 to it so that e.g. monday at 0.00 is its own day.
	// For example, at 6.5 days passed, the number is 7 which is still week 1.
	// However, at exactly 7.0 days (monday at 0.00), the number is Math.floor(7) + 1 i.e. 8, which results in week 2.
	return Math.ceil((Math.floor(dayDiff) + 1) / 7);
}