var todaydate=new Date()
var curmonth=todaydate.getMonth()+1 //get current month (1-12)
var curyear=todaydate.getFullYear() //get current year


function buildCal(m, y, cM, cH, cDW, cD, brdr){
var mn=['January','February','March','April','May','June','July','August','September','October','November','December'];
var dim=[31,0,31,30,31,30,31,31,30,31,30,31];

var oD = new Date(y, m-1, 1); //DD replaced line to fix date bug when current day is 31st
oD.od=oD.getDay()+1; //DD replaced line to fix date bug when current day is 31st

var todaydate=new Date() //DD added
var scanfortoday=(y==todaydate.getFullYear() && m==todaydate.getMonth()+1)? todaydate.getDate() : 0 //DD added

dim[1]=(((oD.getFullYear()%100!=0)&&(oD.getFullYear()%4==0))||(oD.getFullYear()%400==0))?29:28;
var t='<div class="'+cM+'"><table class="'+cM+'" cols="7" cellpadding="0" border="'+brdr+'" cellspacing="0"><tr align="center">';
t+='<td colspan="7" align="center" class="'+cH+'">'+mn[m-1]+' - '+y+'</td></tr><tr align="center">';
for(s=0;s<7;s++)t+='<td class="'+cDW+'">'+"SMTWTFS".substr(s,1)+'</td>';
t+='</tr><tr align="center">';
for(i=1;i<=42;i++){
var x=((i-oD.od>=0)&&(i-oD.od<dim[m-1]))? i-oD.od+1 : '&nbsp;';
if (x==scanfortoday) //DD added
x='<span id="today">'+x+'</span>' //DD added
t+='<td class="'+cD+'">'+x+'</td>';
if(((i)%7==0)&&(i<36))t+='</tr><tr align="center">';
}
return t+='</tr></table></div>';
}
<!-- ende kalender funktion  -->

<!-- beginn position und verschiebung des kalenders  -->
var drag=0;
var offsetx;
var offsety;
document.onmousemove=dragdiv;
document.ondblclick=toggledrag; 
function showdiv(){ document.getElementById('movediv').style.visibility="visible";    
//document.getElementsByName('open')[0].disabled=true;
}

function hidediv(){ document.getElementById('movediv').style.visibility="hidden";    
drag=0;    
document.getElementById('movediv').style.top="3px";    
document.getElementById('movediv').style.left="0px";    
//document.getElementsByName('open')[0].disabled=false;
}
function toggledrag(e){ if (drag==1)        
{drag=0;}    
else if (document.getElementById('movediv').style.visibility=="visible")        
{ posx=document.getElementById('movediv').offsetLeft;            
posy=document.getElementById('movediv').offsetTop;            
divwidth=document.getElementById('movediv').offsetWidth;            
divheight=document.getElementById('movediv').offsetHeight;            
if (document.all){ offsetx=window.event.x;                    
offsety=window.event.y;}            
else{ offsetx=e.pageX;                    
offsety=e.pageY; }            
if ((offsetx>=posx) && (offsetx<=posx+divwidth) && (offsety>=posy) && (offsety<=posy+divheight)) 
{ 
offsetx-=posx;
offsety-=posy;                    
drag=1; }}}
function dragdiv(e){    
if (drag==1){
if (document.all){ mousex=window.event.x;                    
mousey=window.event.y;}            
else{
mousex=e.pageX;                    
mousey=e.pageY;}            
mousex-=offsetx;            
mousey-=offsety;            
document.getElementById('movediv').style.top=mousey+"px";            
document.getElementById('movediv').style.left=mousex+"px";  }}
<!-- ende position und verschiebung des kalenders  -->

month=v_jan*1+v_feb*2+v_mar*3+v_apr*4+v_may*5+v_jun*6+v_jul*7+v_aug*8+v_sep*9+v_oct*10+v_nov*11+v_dec*12;
if (month==0){month = curmonth;};
if (month >12){month = curmonth;};
next_month=month+1; next_year=curyear;
if (month == 12){next_month = 1; next_year = curyear+1};
"showdiv()";