create: function(date) {

  date = date instanceof Date ? date : new Date(date.year, date.month - 1, date.day);

  var inst = Object.create(dateHelper);
  inst._date = date;
  inst._cache = {};

  return inst;
},

get year() {
  this._instanceCheck('year');
  return this._date.getFullYear();
},

set year(y) {
  this._instanceCheck('year');
  this._clearCache();
  return this._date.setFullYear(y);
},

get month() {
  this._instanceCheck('month');
  return this._date.getMonth() + 1;
},

set month(m) {
  this._instanceCheck('month');
  this._clearCache();
  return this._date.setMonth(m - 1);
},

get day() {
  this._instanceCheck('day');
  return this._date.getDate();
},

set day(d) {
  this._instanceCheck('day');
  this._clearCache();
  return this._date.setDate(d);
},

set: function(params) {
  params = params || {};
  this.year = params.year || this.year;
  this.month = params.month || this.month;
  this.day = params.day || this.day;
},

getMonthName: function(num) {
  return monthNames[num - 1];
},

get monthName() {
  this._instanceCheck('monthName');
  return dateHelper.getMonthName(this.month);
},

getMonthNames: function() {
  return monthNames;
},

getMonthNameShort: function(num) {
  return monthNamesShort[num - 1];
},

get monthNameShort() {
  this._instanceCheck('monthName');
  return dateHelper.getMonthNameShort(this.month);
},

getMonthNamesShort: function() {
  return monthNamesShort;
},

setMonthNames: function(names) {
  if (names.length === 12) monthNames = names;
},

setMonthNamesShort: function(names) {
  if (names.length === 12) monthNamesShort = names;
},

getDayOfWeek: function(date) {
  var day = (date instanceof Date ? date : new Date(date.year, date.month - 1, date.day)).getDay() - weekStartsOn;
  return (day < 0 ? 7 - Math.abs(day) : day) + 1;
},

get dayOfWeek() {
  return dateHelper.getDayOfWeek(this._date);
},

getDayName: function(num) {
  return dayNames[num - 1 + weekStartsOn] || dayNames[dayNames.length - num - 1 + weekStartsOn];
},

get dayName() {
  this._instanceCheck('dayName');
  return dateHelper.getDayName(this.dayOfWeek);
},

getDayNames: function() {
  return adjustedDayNames.length ? adjustedDayNames : dayNames;
},

getDayNameShort: function(num) {
  return dayNamesShort[num - 1 + weekStartsOn] || dayNames[dayNames.length - num - 1 + weekStartsOn];
},

get dayNameShort() {
  this._instanceCheck('dayNameShort');
  return dateHelper.getDayNameShort(this.dayOfWeek);
},

getDayNamesShort: function() {
  return adjustedDayNamesShort.length ? adjustedDayNamesShort : dayNamesShort;
},

setDayNames: function(names) {
  if (names.length === 7) dayNames = names;
},

setDayNamesShort: function(names) {
  if (names.length === 7) dayNamesShort = names;
},

getWeekStartsOn: function() {
  return weekStartsOn;
},

setWeekStartsOn: function(number) {

  weekStartsOn = number;

  if (number) {
    adjustedDayNames = dayNames.slice(weekStartsOn);
    adjustedDayNames = adjustedDayNames.concat(dayNames.slice(0, weekStartsOn));
    adjustedDayNamesShort = dayNamesShort.slice(weekStartsOn);
    adjustedDayNamesShort = adjustedDayNamesShort.concat(dayNamesShort.slice(0, weekStartsOn));
  } else {
    adjustedDayNames = [];
    adjustedDayNamesShort = [];
  }
},

now: function() {
  return dateHelper.create(new Date());
},

getNextYear: function(date) {
  return dateHelper.create(new Date(date.year + 1, date.month - 1, date.day));
},

get nextYear() {
  this._instanceCheck('nextYear');
  return this._cache.nextYear || (this._cache.nextYear = dateHelper.getNextYear(this));
},

getWeekStart: function(date) {
  var inst = dateHelper.create(new Date(date.year, date.month - 1, date.day - dateHelper.getDayOfWeek(date) + 1));
  inst.weekStartsOn = weekStartsOn;
  return inst;
},

get weekStart() {
  this._instanceCheck('weekStart');
  return this._cache.weekStart && this._cache.weekStart.weekStartsOn === weekStartsOn ? this._cache.weekStart : (this._cache.weekStart = dateHelper.getWeekStart(this));
},

getMonthStart: function(date) {
  var inst = dateHelper.create(new Date(date.year, date.month - 1, 1));
  return inst;
},

get monthStart() {
  this._instanceCheck('monthStart');
  return this._cache.monthStart || (this._cache.monthStart = dateHelper.getMonthStart(this));
},

getNextWeek: function(date) {
  var start = dateHelper.getWeekStart(date);
  return dateHelper.create(new Date(start.year, start.month - 1, start.day + 7));
},

get nextWeek() {
  this._instanceCheck('nextWeek');
  return this._cache.nextWeek || (this._cache.nextWeek = dateHelper.getNextWeek(this));
},

getNextDay: function(date) {
  return dateHelper.create(new Date(date.year, date.month - 1, date.day + 1));
},

get nextDay() {
  this._instanceCheck('nextDay');
  return this._cache.nextDay || (this._cache.nextDay = dateHelper.getNextDay(this));
},

getNextMonth: function(date) {
  // Date() has a bug if last day of month is 31 when calculating the next month.
  // Need to account for that so that it doesn't round up the date/month.
  return date.day === 31 ? dateHelper.create(new Date(date.year, date.month, 1)) : dateHelper.create(new Date(date.year, date.month, date.day));
},

get nextMonth() {
  this._instanceCheck('nextMonth');
  return this._cache.nextMonth || (this._cache.nextMonth = dateHelper.getNextMonth(this));
},

getPreviousYear: function(date) {
  return dateHelper.create(new Date(date.year - 1, date.month - 1, date.day));
},

get previousYear() {
  this._instanceCheck('previousYear');
  return this._cache.previousYear || (this._cache.previousYear = dateHelper.getPreviousYear(this));
},

getPreviousWeek: function(date) {
  var start = dateHelper.getWeekStart(date);
  var inst = dateHelper.create(new Date(start.year, start.month - 1, start.day - 7));
  inst.weekStartsOn = weekStartsOn;
  return inst;
},

get previousWeek() {
  this._instanceCheck('previousWeek');
  return this._cache.previousWeek || (this._cache.previousWeek = dateHelper.getPreviousWeek(this));
},

getPreviousDay: function(date) {
  return dateHelper.create(new Date(date.year, date.month - 1, date.day - 1));
},

get previousDay() {
  this._instanceCheck('previousDay');
  return this._cache.previousDay || (this._cache.previousDay = dateHelper.getPreviousDay(this));
},

getPreviousMonth: function(date) {
  return dateHelper.create(new Date(date.year, date.month - 2, date.day));
},

get previousMonth() {
  this._instanceCheck('previousMonth');
  return this._cache.previousMonth || (this._cache.previousMonth = dateHelper.getPreviousMonth(this));
},

getMonthEnd: function(date) {
  return dateHelper.create(new Date(date.year, date.month, 0));
},

get monthEnd() {
  this._instanceCheck('monthEnd');
  return this._cache.monthEnd || (this._cache.monthEnd = dateHelper.getMonthEnd(this));
},

equal: function(date, compare, full) {
  return this.equalDay(date, compare, full);
},

equalDay: function(date, compare, full) {

  var args = this._checkComparisonArgs(date, compare, full);
  date = args[0];
  compare = args[1];
  full = args[2];

  var i = 0;
  var len = compare.length;
  var matches = 0;

  for (; i < len; i++) {
    if (compare[i] && date.year === compare[i].year && date.month === compare[i].month && date.day === compare[i].day)
      matches++;
  }

  return full ? matches === len : !!matches;
},

equalWeek: function(date, compare, full) {

  var args = this._checkComparisonArgs(date, compare, full);
  date = args[0];
  compare = args[1];
  full = args[2];

  var i = 0;
  var len = compare.length;
  var matches = 0;

  for (; i < len; i++) {
    if (!compare[i] || !date.weekStart.equalDay(compare[i].weekStart))
      continue;
    else
      matches++;
  }

  return full ? matches === len : !!matches;
},

equalMonth: function(date, compare, full) {

  var args = this._checkComparisonArgs(date, compare, full);
  date = args[0];
  compare = args[1];
  full = args[2];

  var i = 0;
  var len = compare.length;
  var matches = 0;

  for (; i < len; i++) {
    if (!compare[i] || (date.year !== compare[i].year) || (date.year === compare[i].year && date.month !== compare[i].month))
      continue;
    else
      matches++;
  }

  return full ? matches === len : !!matches;
},

equalYear: function(date, compare, full) {

  var args = this._checkComparisonArgs(date, compare, full);
  date = args[0];
  compare = args[1];
  full = args[2];

  var i = 0;
  var len = compare.length;
  var matches = 0;

  for (; i < len; i++) {
    if (!compare[i] || date.year !== compare[i].year)
      continue;
    else
      matches++;
  }

  return full ? matches === len : !!matches;
},

before: function(date, compare, full) {
  return this.beforeDay(date, compare, full);
},

beforeDay: function(date, compare, full) {

  var args = this._checkComparisonArgs(date, compare, full);
  date = args[0];
  compare = args[1];
  full = args[2];

  var i = 0;
  var len = compare.length;
  var matches = 0;

  for (; i < len; i++) {
    if (!compare[i] || date._date >= compare[i]._date)
      continue;
    else
      matches++;
  }

  return full ? matches === len : !!matches;
},

beforeWeek: function(date, compare, full) {

  var args = this._checkComparisonArgs(date, compare, full);
  date = args[0];
  compare = args[1];
  full = args[2];

  var i = 0;
  var len = compare.length;
  var matches = 0;

  for (; i < len; i++) {
    if (!compare[i] || !date.weekStart.beforeDay(compare[i].weekStart))
      continue;
    else
      matches++;
  }

  return full ? matches === len : !!matches;
},

beforeMonth: function(date, compare, full) {

  var args = this._checkComparisonArgs(date, compare, full);
  date = args[0];
  compare = args[1];
  full = args[2];

  var i = 0;
  var len = compare.length;
  var matches = 0;

  for (; i < len; i++) {
    if (!compare[i] || (date.year > compare[i].year) || (date.year === compare[i].year && date.month >= compare[i].month))
      continue;
    else
      matches++;
  }

  return full ? matches === len : !!matches;
},

beforeYear: function(date, compare, full) {

  var args = this._checkComparisonArgs(date, compare, full);
  date = args[0];
  compare = args[1];
  full = args[2];

  var i = 0;
  var len = compare.length;
  var matches = 0;

  for (; i < len; i++) {
    if (!compare[i] || date.year >= compare[i].year)
      continue;
    else
      matches++;
  }

  return full ? matches === len : !!matches;
},

after: function(date, compare, full) {
  return this.afterDay(date, compare, full);
},

afterDay: function(date, compare, full) {

  var args = this._checkComparisonArgs(date, compare, full);
  date = args[0];
  compare = args[1];
  full = args[2];

  var i = 0;
  var len = compare.length;
  var matches = 0;

  for (; i < len; i++) {
    if (!compare[i] || date._date <= compare[i]._date)
      continue;
    else
      matches++;
  }

  return full ? matches === len : !!matches;
},

afterWeek: function(date, compare, full) {

  var args = this._checkComparisonArgs(date, compare, full);
  date = args[0];
  compare = args[1];
  full = args[2];

  var i = 0;
  var len = compare.length;
  var matches = 0;

  for (; i < len; i++) {
    if (!compare[i] || !date.weekStart.afterDay(compare[i].weekStart))
      continue;
    else
      matches++;
  }

  return full ? matches === len : !!matches;
},

afterMonth: function(date, compare, full) {

  var args = this._checkComparisonArgs(date, compare, full);
  date = args[0];
  compare = args[1];
  full = args[2];

  var i = 0;
  var len = compare.length;
  var matches = 0;

  for (; i < len; i++) {
    if (!compare[i] || (date.year < compare[i].year) || (date.year === compare[i].year && date.month <= compare[i].month))
      continue;
    else
      matches++;
  }

  return full ? matches === len : !!matches;
},

afterYear: function(date, compare, full) {

  var args = this._checkComparisonArgs(date, compare, full);
  date = args[0];
  compare = args[1];
  full = args[2];

  var i = 0;
  var len = compare.length;
  var matches = 0;

  for (; i < len; i++) {
    if (!compare[i] || date.year <= compare[i].year)
      continue;
    else
      matches++;
  }

  return full ? matches === len : !!matches;
},

earliest: function(arr) {

  var i = 0;
  var len = arr.length;
  var e;

  for (; i < len; i++) {
    if (!e || arr[i].before(e))
      e = arr[i];
  }

  return e;
},

latest: function(arr) {

  var i = 0;
  var len = arr.length;
  var l;

  for (; i < len; i++) {
    if (!l || arr[i].after(l))
      l = arr[i];
  }

  return l;
},

clone: function(date) {

  // If we weren't passed a date, use this instance.
  if (!date && this._date && this._date instanceof Date && dateHelper.isPrototypeOf(this)) {
    date = this;
  }

  // No date, can't clone.
  if (!date) {
    throw new Error('Must pass a date to clone or call on an instance.');
  }

  return dateHelper.create(new Date(date._date.valueOf()));
},

_checkComparisonArgs: function(date, compare, full) {

  if (compare === undefined || typeof compare === 'boolean') {

    if (!dateHelper.isPrototypeOf(this)) {
      throw new Error('Cannot compare only one date!');
    }

    full = compare;
    compare = date;
    date = this;
  }

  compare = compare instanceof Array ? compare : [compare];

  return [date, compare, full];
},

_instanceCheck: function(prop) {
  if (!this._date || !(this._date instanceof Date) || !dateHelper.isPrototypeOf(this))
    throw new Error('Cannot access the property "' + prop + '" of the date helper with creating an instance!');
},

_clearCache: function() {
  this._instanceCheck('clearCache');
  this._cache = {};
}
  };

  return dateHelper;
}));