My first release of a Mystara Calendar API Script for Roll20.

Roll20 Mystara Calendar

var Calendar = Calendar || {
  version: '0.0.1',
  lunarPhaseSize: 15,
  lunarPhasesImage: 'https://s3.amazonaws.com/files.d20.io/images/4277527/CJJWBbiHx3jHglPdccPx3A/max.png?1401939451',
  clearImage: 'https://s3.amazonaws.com/files.d20.io/images/4277467/iQYjFOsYC5JsuOPUCI9RGA/max.png?1401938659',

  _Ordinal: function(num) {
    var ones=(num%10);
    var tens=((num%100)-ones);
    switch(ones)
    {
      case 1: return ((10 == tens) ? 'th' : 'st');
      case 2: return ((10 == tens) ? 'th' : 'nd');
      case 3: return ((10 == tens) ? 'th' : 'rd');
      default: return 'th';
    }
  },

  _GetOptionsFromTokens: function (tokens) {
    var options={};
    var switches=_.filter(tokens, function(tok){
      return null != tok.match(/^--/);
    });
    _.each(switches,function(s){
      switch(s)
      {
        case '--lunar': options.showLunarPhases=true; break;
        case '--nolunar': options.showLunarPhases=false; break;

      }
    });

    return options;
  },

  CheckInstall: function() {
    log('-=> Calendar v'+Calendar.version+' <=-');


    if( ! state.hasOwnProperty('Calendar') || state.Calendar.version != Calendar.version)
    {
      /* Default Settings stored in the state. */
      state.Calendar = {
        version: Calendar.version,
        now: {
          year: 24789,
          month: 12,
          day: 4
        },
        setting: {
          daysOfTheWeek: [
            'Sunday',
            'Monday',
            'Tuesday',
            'Wednesday',
            'Thursday',
            'Friiday',
            'Saturday'
          ],
          weeksOfTheMonth: [1,2,3,4],
          monthsOfTheYear: [ "January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December" ],
          yearPrefix: 'AC '
        }
      }
    }
  },

  AdvanceDays: function(days){
    var y=Math.floor(days/336);
    days-=(y*336);
    var m = Math.floor(days/28);
    days-=(m*28);

    var n = state.Calendar.now;

    n.day+=days;
    var _m=Math.floor((n.day-1)/28);
    n.day-=(_m*28);
    m+=_m;

    n.month+=m;
    var _y=Math.floor((n.month-1)/12);
    n.month-=(_y*12);
    y+=_y;

    n.year+=y;

    state.Calendar.now=n;
  },

  RemoveDays: function(days){
    var y=Math.floor(days/336);
    days-=(y*336);
    var m = Math.floor(days/28);
    days-=(m*28);

    var n = state.Calendar.now;

    n.day-=days;
    var _m=((n.day>0)?(0):(1));
    n.day+=(_m*28);
    m+=_m;

    n.month-=m;
    var _y=((n.month>0)?(0):(1));
    n.month+=(_y*12);
    y+=_y;

    n.year-=y;

    state.Calendar.now=n;
  },

  _GetPhaseForDate: function(d,options){
    var opt=_.defaults((options||{}),{
      showLunarPhases: true
    });
    return ((opt.showLunarPhases)?(
      '<img src="'
      +Calendar.clearImage
      +'" style="width: '+Calendar.lunarPhaseSize+'px; height: '+Calendar.lunarPhaseSize+'px; background:url('
      +Calendar.lunarPhasesImage
      +') -'+((d.day-1)%7)*Calendar.lunarPhaseSize+'px -'+Math.floor((d.day-1)/7)*Calendar.lunarPhaseSize+'px;">'
    ):(''));
  },

  _GetDayForDate: function(d,options){
    var opt=_.defaults((options||{}),{
    });

    var n=state.Calendar.now;
    var img = Calendar._GetPhaseForDate(d,opt)

    if(d.year == n.year && d.month == n.month && d.day == n.day)
    {
      return '<div style="white-space: nowrap;">'
        +'<span style="font-weight: bold; color: #990000;">'
        +d.day
        +'</span>'
        +img
        +'</div>';
    }
    else if( (d.year < n.year)
      || ( (d.year <= n.year) && (d.month<n.month))
      || ( (d.year <= n.year) && (d.month<=n.month) && (d.day<n.day)) )
    {
      return '<div style="white-space: nowrap;">'
        +'<strike style="color:red; font-weight: bold;">'
        +'<span style="font-weight:bold; color:#999999;">'
        +d.day
        +'</span>'
        +img
        +'</strike>'
        +'</div>';
    }
    else
    {
      return '<div style="white-space: nowrap;">'
        +'<span style="font-weight: bold; color: #000099;">'
        +d.day
        +'</span>'
        +img
        +'</div>';
    }
  },

  _GetMonthForDate: function(d,options){
    var opt=_.defaults((options||{}),{
      showYear: true,
      showMonthNumber: false
    });

    var s=state.Calendar.setting;
    var daysHeader='';
    _.each(s.daysOfTheWeek,function(d){
      daysHeader+='<th><div style="width: 25px;margin: 0px auto;">'+d.substring(0,2)+'</div></th>';
    });

    var mday=_.clone(d);
    var weeks='';
    mday.day=1;
    _.each(s.weeksOfTheMonth,function(w){
      weeks+='<tr>';
      _.each(s.daysOfTheWeek,function(d){
        weeks+='<td style="vertical-align: middle; text-align:right;">';
        weeks+=Calendar._GetDayForDate(mday,opt);
        weeks+='</td>';
        mday.day++;
      });
      weeks+='</tr>';
    });

    return '<table style="border:1px solid black;background-color:#eeffee;">'
      +'<tr><th colspan="'+s.daysOfTheWeek.length+'">'
      +((opt.showMonthNumber)?('<div style="float:right; padding: 0px 3px;">'+d.month+'</div>'):(''))
      +s.monthsOfTheYear[d.month-1]
      +((opt.showYear)?(' '+d.year):(''))
      +'</th></tr>'
      +'<tr style="border-bottom: 1px solid #aaaaaa;">'+daysHeader+'</tr>'
      +weeks
      +'</table>';
  },

  _GetYearForDate: function(d,options){
    var opt=_.defaults((options||{}),{
      showLunarPhases: false,
      showYear: false,
      showMonthNumber: true
    });
    var s=state.Calendar.setting;
    var yday=_.clone(d);
    yday.day=1;
    yday.month=1;
    var months='';
    _.each([1,2,3,4],function(r){
      _.each([1,2,3],function(c){
        months+='<div style="float:left;padding: 2px 2px;">';
        months+=Calendar._GetMonthForDate(yday,opt);
        months+='</div>';
        yday.month++;
      });
    });

    return '<div style="background-color: #DEB887; border: 3px solid #8B4513; padding: 3px 3px;">'
      +'<div style="border-bottom: 2px solid #8B0000;margin: 3px 3px;font-weight: bold; font-size: 130%; text-align: center;">'+s.yearPrefix+d.year+'</div>'
      +months
      +'<div style="clear:both;"></div></div>';
  },

  _GetDateAsString: function(date){
    var s=state.Calendar.setting;
    return s.monthsOfTheYear[date.month-1]+' '+date.day+Calendar._Ordinal(date.day)+', '+s.yearPrefix +date.year;
  },

  ShowDate: function(d,options) {
    var opt=_.defaults((options||{}),{
      showLunarPhases: true
    });
    sendChat('','/direct '
      +'<div style=\''
      +'color: white;'
      +'padding: 5px 5px;'
      +'background-color: #000033;'
      +'font-weight: bold;'
      +'font-family: Baskerville, "Baskerville Old Face", "Goudy Old Style", Garamond, "Times New Roman", serif;'
      +'border: 3px solid #999999;'
      +'text-align: center;'
      +'\'>'
      +Calendar._GetDateAsString(d,opt)
      +' '
      +Calendar._GetPhaseForDate(d,opt)
      +'</div>'
    );
  },

  ShowMonth: function(d,options){
    var opt=_.defaults((options||{}),{
    });
    sendChat('',Calendar._GetMonthForDate(d,opt));
  },

  ShowYear: function(d,options){
    var opt=_.defaults((options||{}),{
      showYear: false
    });
    sendChat('',Calendar._GetYearForDate(d,opt));
  },

  HandleInput: function(tokens){
    var options = Calendar._GetOptionsFromTokens(tokens);
    tokens=_.filter(tokens, function(tok){
      return null == tok.match(/^--/);
    });
    var cmd = tokens[0] || 'month';
    switch (cmd)
    {
      case 'month':
        Calendar.ShowMonth(state.Calendar.now,options);
        break;

      case 'year':
        Calendar.ShowYear(state.Calendar.now,options);
        break;

      case 'today':
        Calendar.ShowDate(state.Calendar.now,options);
        break;

      case 'next':
        var days=tokens[1] || 1;
        Calendar.AdvanceDays(days);
        Calendar.ShowDate(state.Calendar.now,options);
        break;

      case 'prev':
        var days=tokens[1] || 1;
        Calendar.RemoveDays(days);
        Calendar.ShowDate(state.Calendar.now,options);
        break;

    }
  },

  RegisterEventHandlers: function() {
    on("chat:message", function (msg) {
      /* Exit if not an api command */
      if (msg.type != "api") return;

      var tokenized = msg.content.split(/\s+/);
      var command = tokenized[0];

      switch(command)
      {
        case "!cal":
        case "!calendar": {
          Calendar.HandleInput(_.rest(tokenized));
        }
          break;

        case "!s": {
          sendChat('',
            '/direct <div style="border: 2px solid red;"><b>state.Calendar</b><br><pre>'
            +JSON.stringify(state.Calendar,undefined,"   ").replace(/\n/g,'<br>')
            +"</pre></div>" );
        }
          break;
      }
    });
  }
};

on("ready",function(){
  Calendar.CheckInstall();
  Calendar.RegisterEventHandlers();
});