Calendar-related calculations.

date.js

/*
 * Calendar calculations
 * 
 * The following enhancements on the standard JavaScript Date object
 * allow advanced calculations for calendars, especially the
 * calculation of the calendar weeks.
 *
 * @author Karsten Kroesch
 */

Date.prototype.isLeapYear = function() {
  if (this.getFullYear() % 400 == 0) return true
	if (this.getFullYear() % 100 == 0) return false
	if (this.getFullYear() % 4 == 0) return true
	
	return false
}

Date.prototype.getDaysPerMonth = function(month) {
  var days_per_month = new Array(0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31)
	if (month == 2) {
	  if (this.isLeapYear(this.getYear()))
		  return 29
		else
			return 28
	}
	return days_per_month[month]
}

Date.prototype.getDayOfYear = function() {
  var localMonth = this.getMonth()
	var localDay = this.getDay()
	
	while (localMonth > 1) {
	  localMonth --
		localDay += this.getDaysPerMonth(localMonth)
	}
	
	return localDay
}

Date.prototype.getCalendarWeek = function() {
	var jan1 = new Date(this.getYear(), 1, 1)
	var dayJan1 = jan1.getDay()
	var dayOfYear = this.getDayOfYear()
	
	// Exception: Friday and Saturday
	if (dayJan1 >= 4) dayJan1 -= 7
	
	// Exception: Calendar week from year before
	if ((dayOfYear + dayJan1) <= 0)
	  return new Date(this.getYear() - 1, 12, 31).getCalendarWeek()
	
	var calendarWeek = ((dayOfYear - 1 + dayJan1) / 7) + 1
	
	// Years starting on Thursday have 53 weeks except in leap years,
	// where this is also possible for years starting on Wednesday
	if (calendarWeek == 53) {
	  if ((dayJan1 == 3) || ((dayJan1 == 2) && this.isLeapYear())) {
		  ;
		} else {
		  calendarWeek = 1
		}
	}
	
	return Math.round(calendarWeek)
}

Date.prototype.prevMonth = function() {
	this.setMonth(this.getMonth() - 1)
	return this
}

Date.prototype.nextMonth = function() {
	this.setMonth(this.getMonth() + 1)
	return this
}

/**
 * Calculate difference in days between this and another date, 
 * which should be in the future to avoid negative days.
 */
Date.prototype.daysTo = function(date) {
	return Math.round((date.getTime() - this.getTime()) / (1000 * 60 * 60 * 24))
}

Date.prototype.format = function() {
	// NOTE: Month is zero-based, Jan = 0
  return this.getDate() + '.' + (this.getMonth()+1) + '.' + this.getFullYear()
}

Date.prototype.time = function() {
	var padH = this.getHours() > 9 ? '' : '0'
	var padM = this.getMinutes() > 9 ? '' : '0'
	return padH + this.getHours() + ':' + padM + this.getMinutes()
}