jQuery/Zepto Bootstrap Datepicker now with tables and able to start with blank values.

bootstrap.datepicker.js

/* ===========================================================
 * bootstrap-datepicker.js v1.3.0
 * http://twitter.github.com/bootstrap/javascript.html#datepicker
 * ===========================================================
 * Copyright 2011 Twitter, Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 *
 * Contributed by Scott Torborg - github.com/storborg
 * Loosely based on jquery.date_input.js by Jon Leighton, heavily updated and
 * rewritten to match bootstrap javascript approach and add UI features.
 * =========================================================== */


!function ( $ ) { 

  var STANDALONE = true;

  var selector = '[data-datepicker]',
      all = [];

  function clearDatePickers(except) {
    var ii;
    for(ii = 0; ii < all.length; ii++) {
      if(all[ii] != except) {
        all[ii].hide();
      }
    }
  }

  function DatePicker( element, options ) {
    this.$el = $(element);
    this.proxy('show').proxy('ahead').proxy('hide').proxy('keyHandler').proxy('selectDate');

    var options = $.extend({}, $.fn.datepicker.defaults, options );

    if((!!options.parse) || (!!options.format) || !this.detectNative()) {
      $.extend(this, options);
      this.$el.data('datepicker', this);
      all.push(this);
      this.init();
    }
  }

  DatePicker.prototype = {

      detectNative: function(el) {
        if (STANDALONE) return false;
        // Attempt to activate the native datepicker, if there is a known good
        // one. If successful, return true. Note that input type="date"
        // requires that the string be RFC3339, so if the format/parse methods
        // have been overridden, this won't be used.
        if(navigator.userAgent.match(/(iPad|iPhone); CPU(\ iPhone)? OS 5_\d/i)) {
          // jQuery will only change the input type of a detached element.
          var $marker = $('<span>').insertBefore(this.$el);
          this.$el.detach().attr('type', 'date').insertAfter($marker);
          $marker.remove();
          return true;
        }
        return false;
      }

    , init: function() {
        var $months = this.nav('months', 1);
        var $years = this.nav('years', 12);

        var $nav = $('<div>').addClass('nav').append($months).append($years);

        this.$month = $months.find('.name');
        this.$year = $years.find('.name');

        $calendar = $("<table>");
        $dow = $('<tr>');
        $calendar.append($('<thead>'));
        $calendar.find('thead').append($dow);

        // Populate day of week headers, realigned by startOfWeek.
        for (var i = 0; i < this.shortDayNames.length; i++) {
            $dow.append('<th>' + this.shortDayNames[(i + this.startOfWeek) % 7] + '</th>');
        };

        this.$days = $('<tbody>');
        $calendar.append(this.$days);

        this.$picker = $('<div></div>')
          .click(function(e) { e.stopPropagation() })
          // Use this to prevent accidental text selection.
          .mousedown(function(e) { e.preventDefault() })
          .addClass('datepicker')
          .append($nav)
          .append($calendar)
          .insertAfter(this.$el);

        this.$el.change($.proxy(function() { this.selectDate(); }, this));

        this.selectDate();

        if (STANDALONE) {
          this.$el.hide();
          this.show();
        } else {
          this.$el
            .focus(this.show)
            .click(this.show)
          this.hide();
        }
      }

    , nav: function( c, months ) {
        var $subnav = $('<div>' +
                          '<span class="prev button">&larr;</span>' +
                          '<span class="name"></span>' +
                          '<span class="next button">&rarr;</span>' +
                        '</div>').addClass(c)
        $('.prev', $subnav).click($.proxy(function() { this.ahead(-months, 0) }, this));
        $('.next', $subnav).click($.proxy(function() { this.ahead(months, 0) }, this));
        return $subnav;

    }

    , updateName: function($area, s) {
        // Update either the month or year field
        $area.html(s);
    }

    , selectMonth: function(date) {
        var newMonth = new Date(date.getFullYear(), date.getMonth(), 1);

        if (!this.curMonth || !(this.curMonth.getFullYear() == newMonth.getFullYear() &&
                                this.curMonth.getMonth() == newMonth.getMonth())) {

          this.curMonth = newMonth;

          var rangeStart = this.rangeStart(date), rangeEnd = this.rangeEnd(date);
          var num_days = this.daysBetween(rangeStart, rangeEnd);
          this.$days.empty();

          for (var ii = 0; ii <= num_days; ii++) {
            if(ii % 7 == 0) {
                $tr = $('<tr>');
                this.$days.append($tr);
            }
            var thisDay = new Date(rangeStart.getFullYear(), rangeStart.getMonth(), rangeStart.getDate() + ii, 12, 00);
            var $day = $('<td></td>').attr('date', this.format(thisDay));
            $day.text(thisDay.getDate());

            if (thisDay.getMonth() != date.getMonth()) {
              $day.addClass('overlap');
            };

            $tr.append($day);
          };

          this.updateName(this.$month, this.monthNames[date.getMonth()]);
          this.updateName(this.$year, this.curMonth.getFullYear());

          this.$days.find('td').click($.proxy(function(e) {
            var $targ = $(e.target);

            // The date= attribute is used here to provide relatively fast
            // selectors for setting certain date cells.
            this.update($targ.attr("date"));

            this.$days.find('td').removeClass('selected');
            $targ.addClass('selected');
            // Don't consider this selection final if we're just going to an
            // adjacent month.
            if(!$targ.hasClass('overlap')) {
              this.hide();
            }

          }, this));

          $("[date='" + this.format(new Date()) + "']", this.$days).addClass('today');

        };

        // Select the input value if available
        $('.selected', this.$days).removeClass('selected');
        if(this.$el.val()) {
            date = this.parse(this.$el.val());
            selectedDateStr = this.format(date);
            $('[date="' + selectedDateStr + '"]', this.$days).addClass('selected');
        }
      }

    , selectDate: function(date) {
        if (typeof(date) == "undefined") {
            date = this.parse(this.$el.val());
        };
        if (!date) {
            date = new Date();
        }
        this.referenceDate = date;
        this.referenceDateStr = this.format(this.referenceDate);
        this.selectMonth(this.referenceDate);
      }

    , update: function(s) {
        this.$el.val(s);
        this.$el.trigger('change');
      }

    , show: function(e) {
        e && e.stopPropagation();

        // Hide all other datepickers.
        if (!STANDALONE) clearDatePickers(this);

        var offset = this.$el.offset();

        if (!STANDALONE) {
          this.$picker.css({
            top: offset.top + this.$el.outerHeight() + 2,
            left: offset.left,
            position: 'absolute',
            zIndex: '900',
            margin: '0 0 18px 0'
          });
        }
        this.$picker.show();

        if (!STANDALONE) $('html').on('keydown', this.keyHandler);
      }

    , hide: function() {
        if (!STANDALONE) this.$picker.hide();
        if (!STANDALONE) $('html').off('keydown', this.keyHandler);
      }

    , keyHandler: function(e) {

        // Keyboard navigation shortcuts.
        switch (e.keyCode)
        {
          case 9: 
          case 27: 
            // Tab or escape hides the datepicker. In this case, just return
            // instead of breaking, so that the e doesn't get stopped.
            this.hide(); return;
          case 13: 
            // Enter selects the currently highlighted date.
            this.update(this.referenceDateStr); this.hide(); break;
          case 38: 
            // Arrow up goes to prev week.
            this.ahead(0, -7); break;
          case 40: 
            // Arrow down goes to next week.
            this.ahead(0, 7); break;
          case 37: 
            // Arrow left goes to prev day.
            this.ahead(0, -1); break;
          case 39: 
            // Arrow right goes to next day.
            this.ahead(0, 1); break;
          default:
            return;
        }
        e.preventDefault();
      }

    , parse: function(s) {
        // Parse a partial RFC 3339 string into a Date.
        var m;
        if ((m = s.match(/^(\d{4,4})-(\d{2,2})-(\d{2,2})$/))) {
          return new Date(m[1], m[2] - 1, m[3]);
        } else {
          return null;
        }
      }

    , format: function(date) {
        // Format a Date into a string as specified by RFC 3339.
        var month = (date.getMonth() + 1).toString(),
            dom = date.getDate().toString();
        if (month.length === 1) {
          month = '0' + month;
        }
        if (dom.length === 1) {
          dom = '0' + dom;
        }
        return date.getFullYear() + '-' + month + "-" + dom;
      }

    , ahead: function(months, days) {
        // Move ahead ``months`` months and ``days`` days, both integers, can be
        // negative.
        this.selectDate(new Date(this.referenceDate.getFullYear(),
                                 this.referenceDate.getMonth() + months,
                                 this.referenceDate.getDate() + days));
      }

    , proxy: function(meth) {
        // Bind a method so that it always gets the datepicker instance for
        // ``this``. Return ``this`` so chaining calls works.
        this[meth] = $.proxy(this[meth], this);
        return this;
      }

    , daysBetween: function(start, end) {
        // Return number of days between ``start`` Date object and ``end``.
        var start = Date.UTC(start.getFullYear(), start.getMonth(), start.getDate());
        var end = Date.UTC(end.getFullYear(), end.getMonth(), end.getDate());
        return (end - start) / 86400000;
      }

    , findClosest: function(dow, date, direction) {
        // From a starting date, find the first day ahead of behind it that is
        // a given day of the week.
        var difference = direction * (Math.abs(date.getDay() - dow - (direction * 7)) % 7);
        return new Date(date.getFullYear(), date.getMonth(), date.getDate() + difference);
      }

    , rangeStart: function(date) {
        // Get the first day to show in the current calendar view.
        return this.findClosest(this.startOfWeek,
                                new Date(date.getFullYear(), date.getMonth()),
                                -1);
      }

    , rangeEnd: function(date) {
        // Get the last day to show in the current calendar view.
        return this.findClosest((this.startOfWeek - 1) % 7,
                                new Date(date.getFullYear(), date.getMonth() + 1, 0),
                                1);
      }
  };
  
  /* DATEPICKER PLUGIN DEFINITION
   * ============================ */

  $.fn.datepicker = function( options ) {
    return this.each(function() { new DatePicker(this, options); });
  };

  $(function() {
    $(selector).datepicker();
    if (!STANDALONE) {
      $('html').click(clearDatePickers);
    }
  });

  $.fn.datepicker.DatePicker = DatePicker;

  $.fn.datepicker.defaults = {
    monthNames: ["Jan", "Fev", "Mar", "Abr", "Mai", "Jun",
                 "Jul", "Ago", "Set", "Out", "Nov", "Dez"]
  , shortDayNames: ["Dom", "Seg", "Ter", "Qua", "Qui", "Sex", "Sáb"]
  , startOfWeek: 1
  };
}( window.jQuery || window.ender || window.Zepto);