function Calendar(element, instanceOptions) {
	var t = this;



	// Build options object
	// -----------------------------------------------------------------------------------
	// Precedence (lowest to highest): defaults, rtlDefaults, langOptions, instanceOptions

	instanceOptions = instanceOptions || {};

	var options = mergeOptions({}, defaults, instanceOptions);
	var langOptions;

	// determine language options
	if (options.lang in langOptionHash) {
		langOptions = langOptionHash[options.lang];
	}
	else {
		langOptions = langOptionHash[defaults.lang];
	}

	if (langOptions) { // if language options exist, rebuild...
		options = mergeOptions({}, defaults, langOptions, instanceOptions);
	}

	if (options.isRTL) { // is isRTL, rebuild...
		options = mergeOptions({}, defaults, rtlDefaults, langOptions || {}, instanceOptions);
	}


	
	// Exports
	// -----------------------------------------------------------------------------------

	t.options = options;
	t.render = render;
	t.destroy = destroy;
	t.refetchEvents = refetchEvents;
	t.reportEvents = reportEvents;
	t.reportEventChange = reportEventChange;
	t.rerenderEvents = renderEvents; // `renderEvents` serves as a rerender. an API method
	t.changeView = changeView;
	t.select = select;
	t.unselect = unselect;
	t.prev = prev;
	t.next = next;
	t.prevYear = prevYear;
	t.nextYear = nextYear;
	t.today = today;
	t.gotoDate = gotoDate;
	t.incrementDate = incrementDate;
	t.zoomTo = zoomTo;
	t.getDate = getDate;
	t.getCalendar = getCalendar;
	t.getView = getView;
	t.option = option;
	t.trigger = trigger;



	// Language-data Internals
	// -----------------------------------------------------------------------------------
	// Apply overrides to the current language's data


	// Returns moment's internal locale data. If doesn't exist, returns English.
	// Works with moment-pre-2.8
	function getLocaleData(langCode) {
		var f = moment.localeData || moment.langData;
		return f.call(moment, langCode) ||
			f.call(moment, 'en'); // the newer localData could return null, so fall back to en
	}


	var localeData = createObject(getLocaleData(options.lang)); // make a cheap copy

	if (options.monthNames) {
		localeData._months = options.monthNames;
	}
	if (options.monthNamesShort) {
		localeData._monthsShort = options.monthNamesShort;
	}
	if (options.dayNames) {
		localeData._weekdays = options.dayNames;
	}
	if (options.dayNamesShort) {
		localeData._weekdaysShort = options.dayNamesShort;
	}
	if (options.firstDay != null) {
		var _week = createObject(localeData._week); // _week: { dow: # }
		_week.dow = options.firstDay;
		localeData._week = _week;
	}



	// Calendar-specific Date Utilities
	// -----------------------------------------------------------------------------------


	t.defaultAllDayEventDuration = moment.duration(options.defaultAllDayEventDuration);
	t.defaultTimedEventDuration = moment.duration(options.defaultTimedEventDuration);


	// Builds a moment using the settings of the current calendar: timezone and language.
	// Accepts anything the vanilla moment() constructor accepts.
	t.moment = function() {
		var mom;

		if (options.timezone === 'local') {
			mom = fc.moment.apply(null, arguments);

			// Force the moment to be local, because fc.moment doesn't guarantee it.
			if (mom.hasTime()) { // don't give ambiguously-timed moments a local zone
				mom.local();
			}
		}
		else if (options.timezone === 'UTC') {
			mom = fc.moment.utc.apply(null, arguments); // process as UTC
		}
		else {
			mom = fc.moment.parseZone.apply(null, arguments); // let the input decide the zone
		}

		if ('_locale' in mom) { // moment 2.8 and above
			mom._locale = localeData;
		}
		else { // pre-moment-2.8
			mom._lang = localeData;
		}

		return mom;
	};


	// Returns a boolean about whether or not the calendar knows how to calculate
	// the timezone offset of arbitrary dates in the current timezone.
	t.getIsAmbigTimezone = function() {
		return options.timezone !== 'local' && options.timezone !== 'UTC';
	};


	// Returns a copy of the given date in the current timezone of it is ambiguously zoned.
	// This will also give the date an unambiguous time.
	t.rezoneDate = function(date) {
		return t.moment(date.toArray());
	};


	// Returns a moment for the current date, as defined by the client's computer,
	// or overridden by the `now` option.
	t.getNow = function() {
		var now = options.now;
		if (typeof now === 'function') {
			now = now();
		}
		return t.moment(now);
	};


	// Calculates the week number for a moment according to the calendar's
	// `weekNumberCalculation` setting.
	t.calculateWeekNumber = function(mom) {
		var calc = options.weekNumberCalculation;

		if (typeof calc === 'function') {
			return calc(mom);
		}
		else if (calc === 'local') {
			return mom.week();
		}
		else if (calc.toUpperCase() === 'ISO') {
			return mom.isoWeek();
		}
	};


	// Get an event's normalized end date. If not present, calculate it from the defaults.
	t.getEventEnd = function(event) {
		if (event.end) {
			return event.end.clone();
		}
		else {
			return t.getDefaultEventEnd(event.allDay, event.start);
		}
	};


	// Given an event's allDay status and start date, return swhat its fallback end date should be.
	t.getDefaultEventEnd = function(allDay, start) { // TODO: rename to computeDefaultEventEnd
		var end = start.clone();

		if (allDay) {
			end.stripTime().add(t.defaultAllDayEventDuration);
		}
		else {
			end.add(t.defaultTimedEventDuration);
		}

		if (t.getIsAmbigTimezone()) {
			end.stripZone(); // we don't know what the tzo should be
		}

		return end;
	};



	// Date-formatting Utilities
	// -----------------------------------------------------------------------------------


	// Like the vanilla formatRange, but with calendar-specific settings applied.
	t.formatRange = function(m1, m2, formatStr) {

		// a function that returns a formatStr // TODO: in future, precompute this
		if (typeof formatStr === 'function') {
			formatStr = formatStr.call(t, options, localeData);
		}

		return formatRange(m1, m2, formatStr, null, options.isRTL);
	};


	// Like the vanilla formatDate, but with calendar-specific settings applied.
	t.formatDate = function(mom, formatStr) {

		// a function that returns a formatStr // TODO: in future, precompute this
		if (typeof formatStr === 'function') {
			formatStr = formatStr.call(t, options, localeData);
		}

		return formatDate(mom, formatStr);
	};


	
	// Imports
	// -----------------------------------------------------------------------------------


	EventManager.call(t, options);
	ResourceManager.call(t, options);
	var isFetchNeeded = t.isFetchNeeded;
	var fetchEvents = t.fetchEvents;



	// Locals
	// -----------------------------------------------------------------------------------


	var _element = element[0];
	var header;
	var headerElement;
	var content;
	var tm; // for making theme classes
	var currentView;
	var suggestedViewHeight;
	var windowResizeProxy; // wraps the windowResize function
	var ignoreWindowResize = 0;
	var date;
	var events = [];
	
	
	
	// Main Rendering
	// -----------------------------------------------------------------------------------


	if (options.defaultDate != null) {
		date = t.moment(options.defaultDate);
	}
	else {
		date = t.getNow();
	}
	
	
	function render(inc) {
		if (!content) {
			initialRender();
		}
		else if (elementVisible()) {
			// mainly for the public API
			calcSize();
			renderView(inc);
		}
	}
	
	
	function initialRender() {
		tm = options.theme ? 'ui' : 'fc';
		element.addClass('fc');

		if (options.isRTL) {
			element.addClass('fc-rtl');
		}
		else {
			element.addClass('fc-ltr');
		}

		if (options.theme) {
			element.addClass('ui-widget');
		}
		else {
			element.addClass('fc-unthemed');
		}

		content = $("<div class='fc-view-container'/>").prependTo(element);

		header = new Header(t, options);
		headerElement = header.render();
		if (headerElement) {
			element.prepend(headerElement);
		}

		changeView(options.defaultView);

		if (options.handleWindowResize) {
			windowResizeProxy = debounce(windowResize, options.windowResizeDelay); // prevents rapid calls
			$(window).resize(windowResizeProxy);
		}
	}
	
	
	function destroy() {

		if (currentView) {
			currentView.destroy();
		}

		header.destroy();
		content.remove();
		element.removeClass('fc fc-ltr fc-rtl fc-unthemed ui-widget');

		$(window).unbind('resize', windowResizeProxy);
	}
	
	
	function elementVisible() {
		return element.is(':visible');
	}
	
	

	// View Rendering
	// -----------------------------------------------------------------------------------


	function changeView(viewName) {
		renderView(0, viewName);
	}


	// Renders a view because of a date change, view-type change, or for the first time
	function renderView(delta, viewName) {
		ignoreWindowResize++;

		// if viewName is changing, destroy the old view
		if (currentView && viewName && currentView.name !== viewName) {
			header.deactivateButton(currentView.name);
			freezeContentHeight(); // prevent a scroll jump when view element is removed
			if (currentView.start) { // rendered before?
				currentView.destroy();
			}
			currentView.el.remove();
			currentView = null;
		}

		// if viewName changed, or the view was never created, create a fresh view
		if (!currentView && viewName) {
			currentView = new fcViews[viewName](t);
			currentView.el =  $("<div class='fc-view fc-" + viewName + "-view' />").appendTo(content);
			header.activateButton(viewName);
		}

		if (currentView) {

			// let the view determine what the delta means
			if (delta) {
				date = currentView.incrementDate(date, delta);
			}

			// render or rerender the view
			if (
				!currentView.start || // never rendered before
				delta || // explicit date window change
				!date.isWithin(currentView.intervalStart, currentView.intervalEnd) // implicit date window change
			) {
				if (elementVisible()) {

					freezeContentHeight();
					if (currentView.start) { // rendered before?
						currentView.destroy();
					}
					currentView.render(date);
					unfreezeContentHeight();

					// need to do this after View::render, so dates are calculated
					updateTitle();
					updateTodayButton();

					getAndRenderEvents();
				}
			}
		}

		unfreezeContentHeight(); // undo any lone freezeContentHeight calls
		ignoreWindowResize--;
	}
	
	

	// Resizing
	// -----------------------------------------------------------------------------------


	t.getSuggestedViewHeight = function() {
		if (suggestedViewHeight === undefined) {
			calcSize();
		}
		return suggestedViewHeight;
	};


	t.isHeightAuto = function() {
		return options.contentHeight === 'auto' || options.height === 'auto';
	};
	
	
	function updateSize(shouldRecalc) {
		if (elementVisible()) {

			if (shouldRecalc) {
				_calcSize();
			}

			ignoreWindowResize++;
			currentView.updateSize(true); // isResize=true. will poll getSuggestedViewHeight() and isHeightAuto()
			ignoreWindowResize--;

			return true; // signal success
		}
	}


	function calcSize() {
		if (elementVisible()) {
			_calcSize();
		}
	}
	
	
	function _calcSize() { // assumes elementVisible
		if (typeof options.contentHeight === 'number') { // exists and not 'auto'
			suggestedViewHeight = options.contentHeight;
		}
		else if (typeof options.height === 'number') { // exists and not 'auto'
			suggestedViewHeight = options.height - (headerElement ? headerElement.outerHeight(true) : 0);
		}
		else {
			suggestedViewHeight = Math.round(content.width() / Math.max(options.aspectRatio, .5));
		}
	}
	
	
	function windowResize(ev) {
		if (
			!ignoreWindowResize &&
			ev.target === window && // so we don't process jqui "resize" events that have bubbled up
			currentView.start // view has already been rendered
		) {
			if (updateSize(true)) {
				currentView.trigger('windowResize', _element);
			}
		}
	}
	
	
	
	/* Event Fetching/Rendering
	-----------------------------------------------------------------------------*/
	// TODO: going forward, most of this stuff should be directly handled by the view


	function refetchEvents() { // can be called as an API method
		destroyEvents(); // so that events are cleared before user starts waiting for AJAX
		fetchAndRenderEvents();
	}


	function renderEvents() { // destroys old events if previously rendered
		if (elementVisible()) {
			freezeContentHeight();
			currentView.destroyEvents(); // no performance cost if never rendered
			currentView.renderEvents(events);
			unfreezeContentHeight();
		}
	}


	function destroyEvents() {
		freezeContentHeight();
		currentView.destroyEvents();
		unfreezeContentHeight();
	}
	

	function getAndRenderEvents() {
		if (!options.lazyFetching || isFetchNeeded(currentView.start, currentView.end)) {
			fetchAndRenderEvents();
		}
		else {
			renderEvents();
		}
	}


	function fetchAndRenderEvents() {
		fetchEvents(currentView.start, currentView.end);
			// ... will call reportEvents
			// ... which will call renderEvents
	}

	
	// called when event data arrives
	function reportEvents(_events) {
		events = _events;
		renderEvents();
	}


	// called when a single event's data has been changed
	function reportEventChange() {
		renderEvents();
	}



	/* Header Updating
	-----------------------------------------------------------------------------*/


	function updateTitle() {
		header.updateTitle(currentView.title);
	}


	function updateTodayButton() {
		var now = t.getNow();
		if (now.isWithin(currentView.intervalStart, currentView.intervalEnd)) {
			header.disableButton('today');
		}
		else {
			header.enableButton('today');
		}
	}
	


	/* Selection
	-----------------------------------------------------------------------------*/
	

	function select(start, end) {

		start = t.moment(start);
		if (end) {
			end = t.moment(end);
		}
		else if (start.hasTime()) {
			end = start.clone().add(t.defaultTimedEventDuration);
		}
		else {
			end = start.clone().add(t.defaultAllDayEventDuration);
		}

		currentView.select(start, end);
	}
	

	function unselect() { // safe to be called before renderView
		if (currentView) {
			currentView.unselect();
		}
	}
	
	
	
	/* Date
	-----------------------------------------------------------------------------*/
	
	
	function prev() {
		renderView(-1);
	}
	
	
	function next() {
		renderView(1);
	}
	
	
	function prevYear() {
		date.add(-1, 'years');
		renderView();
	}
	
	
	function nextYear() {
		date.add(1, 'years');
		renderView();
	}
	
	
	function today() {
		date = t.getNow();
		renderView();
	}
	
	
	function gotoDate(dateInput) {
		date = t.moment(dateInput);
		renderView();
	}
	
	
	function incrementDate(delta) {
		date.add(moment.duration(delta));
		renderView();
	}


	// Forces navigation to a view for the given date.
	// `viewName` can be a specific view name or a generic one like "week" or "day".
	function zoomTo(newDate, viewName) {
		var viewStr;
		var match;

		if (!viewName || fcViews[viewName] === undefined) { // a general view name, or "auto"
			viewName = viewName || 'day';
			viewStr = header.getViewsWithButtons().join(' '); // space-separated string of all the views in the header

			// try to match a general view name, like "week", against a specific one, like "agendaWeek"
			match = viewStr.match(new RegExp('\\w+' + capitaliseFirstLetter(viewName)));

			// fall back to the day view being used in the header
			if (!match) {
				match = viewStr.match(/\w+Day/);
			}

			viewName = match ? match[0] : 'agendaDay'; // fall back to agendaDay
		}

		date = newDate;
		changeView(viewName);
	}
	
	
	function getDate() {
		return date.clone();
	}



	/* Height "Freezing"
	-----------------------------------------------------------------------------*/


	function freezeContentHeight() {
		content.css({
			width: '100%',
			height: content.height(),
			overflow: 'hidden'
		});
	}


	function unfreezeContentHeight() {
		content.css({
			width: '',
			height: '',
			overflow: ''
		});
	}
	
	
	
	/* Misc
	-----------------------------------------------------------------------------*/
	

	function getCalendar() {
		return t;
	}

	
	function getView() {
		return currentView;
	}
	
	
	function option(name, value) {
		if (value === undefined) {
			return options[name];
		}
		if (name == 'height' || name == 'contentHeight' || name == 'aspectRatio') {
			options[name] = value;
			updateSize(true); // true = allow recalculation of height
		}
	}
	
	
	function trigger(name, thisObj) {
		if (options[name]) {
			return options[name].apply(
				thisObj || _element,
				Array.prototype.slice.call(arguments, 2)
			);
		}
	}

}