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 = rerenderEvents;
	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.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

	var langData = createObject( // make a cheap clone
		moment.localeData(options.lang)
	);

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

		mom._lang = langData;

		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, langData);
		}

		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, langData);
		}

		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 elementOuterWidth;
	var suggestedViewHeight;
	var resizeUID = 0;
	var ignoreWindowResize = 0;
	var date;
	var events = [];
	var _dragElement;
	
	
	
	// 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');
		}

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

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

		changeView(options.defaultView);

		if (options.handleWindowResize) {
			$(window).resize(windowResize);
		}

		// needed for IE in a 0x0 iframe, b/c when it is resized, never triggers a windowResize
		if (!bodyVisible()) {
			lateRender();
		}
	}
	
	
	// called when we know the calendar couldn't be rendered when it was initialized,
	// but we think it's ready now
	function lateRender() {
		setTimeout(function() { // IE7 needs this so dimensions are calculated correctly
			if (!currentView.start && bodyVisible()) { // !currentView.start makes sure this never happens more than once
				renderView();
			}
		},0);
	}
	
	
	function destroy() {

		if (currentView) {
			trigger('viewDestroy', currentView, currentView, currentView.element);
			currentView.triggerEventDestroy();
		}

		$(window).unbind('resize', windowResize);

		if (options.droppable) {
			$(document)
				.off('dragstart', droppableDragStart)
				.off('dragstop', droppableDragStop);
		}

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

		header.destroy();
		content.remove();
		element.removeClass('fc fc-ltr fc-rtl ui-widget');
	}
	
	
	function elementVisible() {
		return element.is(':visible');
	}
	
	
	function bodyVisible() {
		return $('body').is(':visible');
	}
	
	

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

	function changeView(newViewName) {
		if (!currentView || newViewName != currentView.name) {
			_changeView(newViewName);
		}
	}


	function _changeView(newViewName) {
		ignoreWindowResize++;

		if (currentView) {
			trigger('viewDestroy', currentView, currentView, currentView.element);
			unselect();
			currentView.triggerEventDestroy(); // trigger 'eventDestroy' for each event
			freezeContentHeight();
			currentView.element.remove();
			header.deactivateButton(currentView.name);
		}

		header.activateButton(newViewName);

		currentView = new fcViews[newViewName](
			$("<div class='fc-view fc-view-" + newViewName + "' />")
				.appendTo(content),
			t // the calendar object
		);

		renderView();
		unfreezeContentHeight();

		ignoreWindowResize--;
	}


	function renderView(inc) {
		if (
			!currentView.start || // never rendered before
			inc || // explicit date window change
			!date.isWithin(currentView.intervalStart, currentView.intervalEnd) // implicit date window change
		) {
			if (elementVisible()) {
				_renderView(inc);
			}
		}
	}


	function _renderView(inc) { // assumes elementVisible
		ignoreWindowResize++;

		if (currentView.start) { // already been rendered?
			trigger('viewDestroy', currentView, currentView, currentView.element);
			unselect();
			clearEvents();
		}

		freezeContentHeight();
		if (inc) {
			date = currentView.incrementDate(date, inc);
		}
		currentView.render(date.clone()); // the view's render method ONLY renders the skeleton, nothing else
		setSize();
		unfreezeContentHeight();
		(currentView.afterRender || noop)();

		updateTitle();
		updateTodayButton();

		trigger('viewRender', currentView, currentView, currentView.element);

		ignoreWindowResize--;

		getAndRenderEvents();
	}
	
	

	// Resizing
	// -----------------------------------------------------------------------------------
	
	
	function updateSize() {
		if (elementVisible()) {
			unselect();
			clearEvents();
			calcSize();
			setSize();
			renderEvents();
		}
	}
	
	
	function calcSize() { // assumes elementVisible
		if (options.contentHeight) {
			suggestedViewHeight = options.contentHeight;
		}
		else if (options.height) {
			suggestedViewHeight = options.height - (headerElement ? headerElement.height() : 0) - vsides(content);
		}
		else {
			suggestedViewHeight = Math.round(content.width() / Math.max(options.aspectRatio, .5));
		}
	}
	
	
	function setSize() { // assumes elementVisible

		if (suggestedViewHeight === undefined) {
			calcSize(); // for first time
				// NOTE: we don't want to recalculate on every renderView because
				// it could result in oscillating heights due to scrollbars.
		}

		ignoreWindowResize++;
		currentView.setHeight(suggestedViewHeight);
		currentView.setWidth(content.width());
		ignoreWindowResize--;

		elementOuterWidth = element.outerWidth();
	}
	
	
	function windowResize(ev) {
		if (
			!ignoreWindowResize &&
			ev.target === window // so we don't process jqui "resize" events that have bubbled up
		) {
			if (currentView.start) { // view has already been rendered
				var uid = ++resizeUID;
				setTimeout(function() { // add a delay
					if (uid == resizeUID && !ignoreWindowResize && elementVisible()) {
						if (elementOuterWidth != (elementOuterWidth = element.outerWidth())) {
							ignoreWindowResize++; // in case the windowResize callback changes the height
							updateSize();
							currentView.trigger('windowResize', _element);
							ignoreWindowResize--;
						}
					}
				}, options.windowResizeDelay);
			}else{
				// calendar must have been initialized in a 0x0 iframe that has just been resized
				lateRender();
			}
		}
	}
	
	
	
	/* 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
		clearEvents();
		fetchAndRenderEvents();
	}


	function rerenderEvents(modifiedEventID) { // can be called as an API method
		clearEvents();
		renderEvents(modifiedEventID);
	}


	function renderEvents(modifiedEventID) { // TODO: remove modifiedEventID hack
		if (elementVisible()) {
			currentView.renderEvents(events, modifiedEventID); // actually render the DOM elements
			currentView.trigger('eventAfterAllRender');
		}
	}


	function clearEvents() {
		currentView.triggerEventDestroy(); // trigger 'eventDestroy' for each event
		currentView.clearEvents(); // actually remove the DOM elements
		currentView.clearEventData(); // for View.js, TODO: unify with clearEvents
	}
	

	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(eventID) {
		rerenderEvents(eventID);
	}



	/* 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) {
		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('years', -1);
		renderView();
	}
	
	
	function nextYear() {
		date.add('years', 1);
		renderView();
	}
	
	
	function today() {
		date = t.getNow();
		renderView();
	}
	
	
	function gotoDate(dateInput) {
		date = t.moment(dateInput);
		renderView();
	}
	
	
	function incrementDate(delta) {
		date.add(moment.duration(delta));
		renderView();
	}
	
	
	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();
		}
	}
	
	
	function trigger(name, thisObj) {
		if (options[name]) {
			return options[name].apply(
				thisObj || _element,
				Array.prototype.slice.call(arguments, 2)
			);
		}
	}
	
	
	
	/* External Dragging
	------------------------------------------------------------------------*/
	
	if (options.droppable) {
		// TODO: unbind on destroy
		$(document)
			.on('dragstart', droppableDragStart)
			.on('dragstop', droppableDragStop);
		// this is undone in destroy
	}

	function droppableDragStart(ev, ui) {
		var _e = ev.target;
		var e = $(_e);
		if (!e.parents('.fc').length) { // not already inside a calendar
			var accept = options.dropAccept;
			if ($.isFunction(accept) ? accept.call(_e, e) : e.is(accept)) {
				_dragElement = _e;
				currentView.dragStart(_dragElement, ev, ui);
			}
		}
	}

	function droppableDragStop(ev, ui) {
		if (_dragElement) {
			currentView.dragStop(_dragElement, ev, ui);
			_dragElement = null;
		}
	}
	

}